.. _library_diagrams:

``diagrams``
============

This tool generates *library*, *directory*, *file*, *entity*, and
*predicate* diagrams for source files and for libraries of source files
using the Logtalk reflection API to collect the relevant information and
a graph language for representing the diagrams. Limited support is also
available for generating diagrams for Prolog module applications. It's
also possible in general to generate predicate cross-referencing
diagrams for plain Prolog files.

Linking *library diagrams* to *entity diagrams* to *predicate
cross-referencing diagrams* and linking *directory diagrams* to *file
diagrams* is also supported when using SVG output. This feature allows
using diagrams for understanding the architecture of applications by
navigating complex code and zooming into details. SVG output can also
easily link to both source code repositories and API documentation. This
allows diagrams to be used for source code navigation.

Diagrams can also be used to uncover code issues. For example, comparing
*loading diagrams* with *dependency diagrams* can reveal implicit
dependencies. Loading diagrams can reveal circular dependencies that may
warrant code refactoring. Entity diagrams can provide a good overview of
code coupling. Predicate cross-referencing diagrams can be used to
visually access entity code complexity, complementing the
``code_metrics`` tool.

All diagrams support a comprehensive set of options, discussed below, to
customize the final contents and appearance.

Diagram generation can be easily automated using the ``doclet`` tool and
the ``logtalk_doclet`` scripts. See the ``doclet`` tool examples and
documentation for details. See also the ``diagrams`` tool own
``lgt2svg`` Bash and PowerShell scripts.

Requirements
------------

Recent versions of d2 (0.6.9 or later) or Graphviz are required for
generating diagrams in the final formats:

- https://d2lang.com
- https://www.graphviz.org/

These can be installed using their own installers or using
operating-system package managers:

macOS - MacPorts
~~~~~~~~~~~~~~~~

::

   $ sudo port install d2 graphviz

macOS - Homebrew
~~~~~~~~~~~~~~~~

::

   $ brew install d2 graphviz

Windows - Chocolatey
~~~~~~~~~~~~~~~~~~~~

::

   > choco install d2 graphviz

Installers
~~~~~~~~~~

- https://d2lang.com/tour/install
- https://www.graphviz.org/download/

API documentation
-----------------

This tool API documentation is available at:

`../../apis/library_index.html#diagrams <../../apis/library_index.html#diagrams>`__

For sample queries, please see the ``SCRIPT.txt`` file in the tool
directory.

Loading
-------

This tool can be loaded using the query:

::

   | ?- logtalk_load(diagrams(loader)).

Testing
-------

To test this tool, load the ``tester.lgt`` file:

::

   | ?- logtalk_load(diagrams(tester)).

Supported diagrams
------------------

The following entity diagrams are supported:

- *entity diagrams* showing entity public interfaces, entity inheritance
  relations, and entity predicate cross-reference relations
- *predicate cross-reference diagrams* (between entities or within an
  entity)
- *inheritance diagrams* showing entity inheritance relations
- *uses diagrams* showing which entities use resources from other
  entities

The following library diagrams are supported:

- *library loading diagrams* showing which libraries load other
  libraries
- *library dependency diagrams* showing which libraries contain entities
  with references to entities defined in other libraries

The following file diagrams are supported:

- *file loading diagrams* showing which files load or include other
  files
- *file dependency diagrams* showing which files contain entities with
  references to entities defined in other files

File dependency diagrams are specially useful in revealing dependencies
that are not represented in file loading diagrams due to files being
loaded indirectly by files external to the libraries being documented.

The following directory diagrams are supported:

- *directory loading diagrams* showing which directories contain files
  that load files from other directories
- *directory dependency diagrams* showing which directories contain
  entities with references to entities defined in other directories

Comparing directory (or file) loading diagrams with directory (or file)
dependency diagrams allows comparing what is explicitly loaded with the
actual directory (or file) dependencies, which are inferred from the
source code.

Library and directory dependency diagrams are specially useful for large
applications where file diagrams would be too large and complex to be
useful, notably when combined with the *zoom* option to link to,
respectively, entity and file diagrams.

A utility object, ``diagrams``, is provided for generating all supported
diagrams in one step. This object provides an interface common to all
diagrams, but note that some predicates that generate diagrams only make
sense for some types of diagrams. For best results and fine-grained
customization of each diagram, the individual diagram objects should be
used with the intended set of options.

Graph elements
--------------

Limitations in both the graph languages and UML force the invention of a
modeling language that can represent all kinds of Logtalk entities and
entity relations. Currently we use the following Graphviz node shapes
(libraries, entities, predicates, and files) and arrowheads (entity,
predicate, and file relations):

- | libraries
  | ``tab`` (lightsalmon)

- | library loading and dependency relations
  | ``normal`` (arrow ending with a black triangle)

- | objects (classes, instances, and prototypes)
  | ``box`` (rectangle, yellow for instances/classes and beige for
    prototypes)

- | protocols
  | ``note`` (aqua marine rectangle with folded right-upper corners)

- | categories
  | ``component`` (light cyan rectangle with two small rectangles
    intercepting the left side)

- | modules
  | ``box`` (plum rectangle with small tab at top)

- | public predicates
  | ``box`` (springgreen)

- | public, multifile, predicates
  | ``box`` (skyblue)

- | protected predicates
  | ``box`` (yellow)

- | private predicates
  | ``box`` (indianred)

- | external predicates
  | ``box`` (beige)

- | exported module predicates
  | ``box`` (springgreen)

- | directories
  | ``tab`` (lightsalmon)

- | directory loading and dependency relations
  | ``normal`` (arrow ending with a black triangle)

- | files
  | ``box`` (pale turquoise rectangle)

- | file loading and dependency relations
  | ``normal`` (arrow ending with a black triangle)

- | specialization relation
  | ``onormal`` (arrow ending with a white triangle)

- | instantiation relation
  | ``normal`` (arrow ending with a black triangle)

- | extends relation
  | ``vee`` (arrow ending with a "v")

- | implements relation
  | ``dot`` (arrow ending with a black circle)

- | imports relation
  | ``box`` (arrow ending with a black square)

- | complements relation
  | ``obox`` (arrow ending with a white square)

- | uses and use module relations
  | ``rdiamond`` (arrow ending with a black half diamond)

- | predicate calls
  | ``normal`` (arrow ending with a black triangle)

- | dynamic predicate updates
  | ``diamond`` (arrow ending with a black diamond)

When using the d2 graph language, we use similar node shapes and
arrowheads when available. As d2 evolves, we hope that these graph
elements will converge further.

The library, directory, file, entity, and predicate nodes that are not
part of the predicates, entities, files, or libraries for which we are
generating a diagram use a dashed border, a darker color, and are
described as external.

Note that all the elements above can have captions. See below the
diagrams ``node_type_captions/1`` and ``relation_labels/1`` output
options.

Supported graph languages
-------------------------

Currently, both DOT and d2 graph languages support all the features of
the ``diagrams`` tool. There's also preliminary support for Mermaid (not
loaded by default, as its current version lacks several required
features for parity with d2 and Graphviz).

The diagrams ``.d2`` and ``.dot`` files are created in the current
directory by default. These files can be easily converted into a
printable format such as SVG, PDF, or Postscript. Sample helper scripts
are provided for converting a directory of ``.d2`` or ``.dot`` files to
``.svg`` files:

- ``lgt2svg.sh`` for POSIX systems
- ``lgt2svg.ps1`` for Windows systems

The scripts assume that the d2 and Graphviz command-line executables are
available from the system path. For Graphviz, the default is the ``dot``
executable, but the scripts accept a command-line option to select in
alternative the ``circo``, ``fdp``, or ``neato`` executables). For d2,
the default layout engine is ``elk``, but the scripts accept a
command-line option to select in alternative the ``dagre`` or ``tala``
layout engines.

The recommended output format is SVG, as it supports tooltips and URL
links, which can be used for showing, e.g., entity types, relation
types, file paths, and for navigating to files and directories of files
(libraries) or to API documentation. See the relevant diagram options
below in order to take advantage of these features (see the discussion
below on "linking diagrams").

To convert to formats other than SVG, you will need to use the d2 and
Graphviz executables directly. For example, using the Graphviz ``dot``
executable, we can generate a PDF with the command:

::

   dot -Tpdf diagram.dot > diagram.pdf

This usually works fine for entity and predicate call cross-referencing
diagrams. For directory and file diagrams, the ``fdp`` and ``circo``
command-line executables may produce better results. For example:

::

   fdp -Tsvg diagram.dot > diagram.svg
   circo -Tsvg diagram.dot > diagram.svg

It's also worth experimenting with different layouts to find the one
that produces the best results (see the ``layout/1`` option described
below).

When generating diagrams for multiple libraries or directories, it's
possible to split a diagram with several disconnected library or
directory graphs using the ``ccomps`` command-line executable. For
example:

::

   ccomps -x -o subdiagram.dot diagram.dot

For more information on the DOT language and related tools, see:

::

   http://www.graphviz.org/

When using Windows, there are known issues with some Prolog compilers
due to the internal representation of paths. If you encounter problems
with a specific backend Prolog compiler, try, if possible, to use
another supported backend Prolog compiler when generating diagrams.

For printing large diagrams, you will need to either use a tool to slice
the diagram into page-sized pieces or, preferably, use software capable
of tiled printing (e.g., Adobe Reader). You can also hand-edit the
generated ``.dot`` files and play with settings such as aspect ratio for
fine-tuning the diagrams layout.

Customization
-------------

A set of options is available to specify the details to include in the
generated diagrams. For entity diagrams, the options are:

- | ``layout(Layout)``
  | diagram layout (one of the atoms
    ``{top_to_bottom,bottom_to_top,left_to_right,right_to_left}``;
    default is ``bottom_to_top``)

- | ``title(Title)``
  | diagram title (an atom; default is ``''``)

- | ``date(Boolean)``
  | print current date and time (``true`` or ``false``; default is
    ``true``)

- | ``versions(Boolean)``
  | print Logtalk and backend version data (``true`` or ``false``;
    default is ``false``)

- | ``interface(Boolean)``
  | print public predicates (``true`` or ``false``; default is ``true``)

- | ``file_labels(Boolean)``
  | print file labels (``true`` or ``false``; default is ``true``)

- | ``file_extensions(Boolean)``
  | print file name extensions (``true`` or ``false``; default is
    ``true``)

- | ``relation_labels(Boolean)``
  | print entity relation labels (``true`` or ``false``; default is
    ``true``)

- | ``externals(Boolean)``
  | print external nodes (``true`` or ``false``; default is ``true``)

- | ``node_type_captions(Boolean)``
  | print node type captions (``true`` or ``false``; default is
    ``true``)

- | ``inheritance_relations(Boolean)``
  | print inheritance relations (``true`` or ``false``; default is
    ``true`` for entity inheritance diagrams and ``false`` for other
    entity diagrams)

- | ``provide_relations(Boolean)``
  | print provide relations (``true`` or ``false``; default is
    ``false``)

- | ``xref_relations(Boolean)``
  | print predicate call cross-reference relations (``true`` or
    ``false``; default depends on the specific diagram)

- | ``xref_calls(Boolean)``
  | print predicate cross-reference calls (``true`` or ``false``;
    default depends on the specific diagram)

- | ``output_directory(Directory)``
  | directory for the ``.d2`` and ``.dot`` files (an atom; default is
    ``'./dot_dias'``)

- | ``exclude_directories(Directories)``
  | list of directories to exclude except as external nodes (default is
    ``[]``); all sub-directories of the excluded directories are also
    excluded; directories may be listed by full or relative path

- | ``exclude_files(Files)``
  | list of source files to exclude except as external nodes (default is
    ``[]``); files may be listed by full path or basename, with or
    without extension

- | ``exclude_libraries(Libraries)``
  | list of libraries to exclude except as external nodes (default is
    ``[startup, scratch_directory]``)

- | ``exclude_entities(Entities)``
  | list of entities to exclude except as external nodes (default is
    ``[]``)

- | ``path_url_prefixes(PathPrefix, CodeURLPrefix, DocURLPrefix)``
  | code and documenting URL prefixes for a path prefix used when
    generating cluster, library, directory, file, and entity links
    (atoms; no default; can be specified multiple times)

- | ``url_prefixes(CodeURLPrefix, DocURLPrefix)``
  | default URL code and documenting URL prefixes used when generating
    cluster, library, file, and entity links (atoms; no default)

- | ``entity_url_suffix_target(Suffix, Target)``
  | extension for entity documenting URLs (an atom; default is
    ``'.html'``) and target separating symbols (an atom; default is
    ``'#'``)

- | ``omit_path_prefixes(Prefixes)``
  | omit common path prefixes when printing directory paths and when
    constructing URLs (a list of atoms; default is a list with the user
    home directory)

- | ``zoom(Boolean)``
  | generate sub-diagrams and add links and zoom icons to library and
    entity nodes (``true`` or ``false``; default is ``false``)

- | ``zoom_url_suffix(Suffix)``
  | extension for linked diagrams (an atom; default is ``'.svg'``)

In the particular case of cross-referencing diagrams, there are also the
options:

- | ``recursive_relations(Boolean)``
  | print recursive predicate relations (``true`` or ``false``; default
    is ``false``)

- | ``url_line_references(Host)``
  | syntax for the URL source file line part (an atom; possible values
    are ``{github,gitlab,bitbucket}``; default is ``github``); when
    using this option, the ``CodeURLPrefix`` should be a permanent link
    (i.e., it should include the commit SHA1)

- | ``predicate_url_target_format(Generator)``
  | documentation final format generator (an atom; default is
    ``sphinx``)

For directory and file diagrams, the options are:

- | ``layout(Layout)``
  | diagram layout (one of the atoms
    ``{top_to_bottom,bottom_to_top,left_to_right,right_to_left}``;
    default is ``top_to_bottom``)

- | ``title(Title)``
  | diagram title (an atom; default is ``''``)

- | ``date(Boolean)``
  | print current date and time (``true`` or ``false``; default is
    ``true``)

- | ``versions(Boolean)``
  | print Logtalk and backend version data (``true`` or ``false``;
    default is ``false``)

- | ``directory_paths(Boolean)``
  | print file directory paths (``true`` or ``false``; default is
    ``false``)

- | ``file_extensions(Boolean)``
  | print file name extensions (``true`` or ``false``; default is
    ``true``)

- | ``path_url_prefixes(PathPrefix, CodeURLPrefix, DocURLPrefix)``
  | code and documenting URL prefixes for a path prefix used when
    generating cluster, directory, file, and entity links (atoms; no
    default; can be specified multiple times)

- | ``url_prefixes(CodeURLPrefix, DocURLPrefix)``
  | default URL code and documenting URL prefixes used when generating
    cluster, library, file, and entity links (atoms; no default)

- | ``omit_path_prefixes(Prefixes)``
  | omit common path prefixes when printing directory paths and when
    constructing URLs (a list of atoms; default is a list with the user
    home directory)

- | ``relation_labels(Boolean)``
  | print entity relation labels (``true`` or ``false``; default is
    ``false``)

- | ``externals(Boolean)``
  | print external nodes (``true`` or ``false``; default is ``true``)

- | ``node_type_captions(Boolean)``
  | print node type captions (``true`` or ``false``; default is
    ``false``)

- | ``output_directory(Directory)``
  | directory for the ``.d2`` and ``.dot`` files (an atom; default is
    ``'./dot_dias'``)

- | ``exclude_directories(Directories)``
  | list of directories to exclude except as external nodes (default is
    ``[]``)

- | ``exclude_files(Files)``
  | list of source files to exclude except as external nodes (default is
    ``[]``)

- | ``zoom(Boolean)``
  | generate sub-diagrams and add links and zoom icons to library and
    entity nodes (``true`` or ``false``; default is ``false``)

- | ``zoom_url_suffix(Suffix)``
  | extension for linked diagrams (an atom; default is ``'.svg'``)

For library diagrams, the options are:

- | ``layout(Layout)``
  | diagram layout (one of the atoms
    ``{top_to_bottom,bottom_to_top,left_to_right,right_to_left}``;
    default is ``top_to_bottom``)

- | ``title(Title)``
  | diagram title (an atom; default is ``''``)

- | ``date(Boolean)``
  | print current date and time (``true`` or ``false``; default is
    ``true``)

- | ``versions(Boolean)``
  | print Logtalk and backend version data (``true`` or ``false``;
    default is ``false``)

- | ``directory_paths(Boolean)``
  | print file directory paths (``true`` or ``false``; default is
    ``false``)

- | ``path_url_prefixes(PathPrefix, CodeURLPrefix, DocURLPrefix)``
  | code and documenting URL prefixes for a path prefix used when
    generating cluster, library, file, and entity links (atoms; no
    default; can be specified multiple times)

- | ``url_prefixes(CodeURLPrefix, DocURLPrefix)``
  | default URL code and documenting URL prefixes used when generating
    cluster, library, file, and entity links (atoms; no default)

- | ``omit_path_prefixes(Prefixes)``
  | omit common path prefixes when printing directory paths and when
    constructing URLs (a list of atoms; default is a list with the user
    home directory)

- | ``relation_labels(Boolean)``
  | print entity relation labels (``true`` or ``false``; default is
    ``false``)

- | ``externals(Boolean)``
  | print external nodes (``true`` or ``false``; default is ``true``)

- | ``node_type_captions(Boolean)``
  | print node type captions (``true`` or ``false``; default is
    ``false``)

- | ``output_directory(Directory)``
  | directory for the ``.d2`` and ``.dot`` files (an atom; default is
    ``'./dot_dias'``)

- | ``exclude_directories(Directories)``
  | list of directories to exclude except as external nodes (default is
    ``[]``)

- | ``exclude_files(Files)``
  | list of source files to exclude except as external nodes (default is
    ``[]``)

- | ``exclude_libraries(Libraries)``
  | list of libraries to exclude except as external nodes (default is
    ``[startup, scratch_directory]``)

- | ``zoom(Boolean)``
  | generate sub-diagrams and add links and zoom icons to library and
    entity nodes (``true`` or ``false``; default is ``false``)

- | ``zoom_url_suffix(Suffix)``
  | extension for linked diagrams (an atom; default is ``'.svg'``)

When using the ``zoom(true)`` option, the ``layout(Layout)`` option
applies only to the top diagram; sub-diagrams will use their own layout
default.

The option ``omit_path_prefixes(Prefixes)`` with a non-empty list of
prefixes should preferably be used together with the option
``directory_paths(true)`` when generating library or file diagrams that
reference external libraries or files. To confirm the exact default
options used by each type of diagram, send the ``default_options/1``
message to the diagram object.

Be sure to set the ``source_data`` flag ``on`` before compiling the
libraries or files for which you want to generate diagrams.

Support for displaying Prolog modules and Prolog module files in
diagrams of Logtalk applications:

- | ECLiPSe
  | file diagrams don't display module files

- | SICStus Prolog
  | file diagrams don't display module files

- | SWI-Prolog
  | full support (uses the SWI-Prolog ``prolog_xref`` library)

- | YAP
  | full support (uses the YAP ``prolog_xref`` library)

Linking diagrams
----------------

When using SVG output, it's possible to generate diagrams that link to
other diagrams, to API documentation, to local files and directories,
and to source code repositories.

For generating links between diagrams, use the ``zoom(true)`` option.
This option allows (1) linking library diagrams to entity diagrams to
predicate cross-referencing diagrams and (2) linking directory diagrams
to file diagrams to entity diagrams to predicate cross-referencing
diagrams. The sub-diagrams are automatically generated. For example,
using the predicates that generate library diagrams will also
automatically generate the entity and predicate cross-referencing
diagrams.

To generate local links for opening directories, files, and file
locations in selected text editors, set the URL code prefix:

- VSCode: ``url_prefixes('vscode://file/', DocPrefix)``
- VSCodium: ``url_prefixes('vscodium://file/', DocPrefix)``
- Cursor: ``url_prefixes('cursor://file/', DocPrefix)``
- PearAI: ``url_prefixes('pearai://file/', DocPrefix)``
- Windsurf: ``url_prefixes('windsurf://file/', DocPrefix)``
- Zed: ``url_prefixes('zed://file/', DocPrefix)``
- BBEdit: ``url_prefixes('x-bbedit://open?url=file://', DocPrefix)``
- MacVim: ``url_prefixes('mvim://open?url=file://', DocPrefix)``
- TextMate: ``url_prefixes('txmt://open?url=file://', DocPrefix)``
- IDEA: ``url_prefixes('idea://open?file=', DocPrefix)``
- PyCharm: ``url_prefixes('pycharm://open?file=', DocPrefix)``

In this case, the ``DocPrefix`` argument should be the path to the
directory containing the HTML version of the application APIs.

As most of the text editor URL scheme handlers require local links to
use absolute paths, the ``omit_path_prefixes/1`` option is ignored. Note
that local links require text editor support for URL schemes that can
handle both file and directory links.

To generate links to API documentation and source code repositories, use
the options ``path_url_prefixes/3`` (or ``url_prefixes/2`` for simpler
cases) and ``omit_path_prefixes/1``. The idea is that the
``omit_path_prefixes/1`` option specifies local file prefixes that will
be cut and replaced by the URL prefixes (which can be path prefix
specific when addressing multiple code repositories). To generate local
file system URLs, define the empty atom, ``''``, as a prefix. As an
example, consider the Logtalk library. Its source code is available from
a GitHub repository, and its documentation is published on the Logtalk
website. The relevant URLs in this case are:

- https://github.com/LogtalkDotOrg/logtalk3/ (source code)
- https://logtalk.org/library/ (API documentation)

Git source code URLs should include the commit SHA1 to ensure that
entity and predicate file line information in the URLs remain valid if
the code changes in later commits. Assuming a ``GitHub`` variable bound
to the SHA1 commit URL we want to reference, an inheritance diagram can
be generated using the goal:

::

   | ?- GitHub  = 'https://github.com/LogtalkDotOrg/logtalk3/commit/eb156d46e135ac47ef23adcc5d20d49dd8b66abb',
        APIDocs = 'https://logtalk.org/library/',
        logtalk_load(diagrams(loader)),
        set_logtalk_flag(source_data, on),
        logtalk_load(library(all_loader)),
        inheritance_diagram::rlibrary(library, [
            title('Logtalk library'),
            node_type_captions(true),
            zoom(true),
            path_url_prefixes('$LOGTALKUSER/', GitHub, APIDocs),
            path_url_prefixes('$LOGTALKHOME/', GitHub, APIDocs),
            omit_path_prefixes(['$LOGTALKUSER/', '$LOGTALKHOME/', '$HOME/'])
        ]).

The two ``path_url_prefixes/3`` options take care of source code and API
documentation for entities loaded either from the Logtalk installation
directory (whose location is given by the ``LOGTALKHOME`` environment
variable) or from the Logtalk user directory (whose location is given by
the ``LOGTALKUSER`` environment variable). As we also don't want any
local operating-system paths to be exposed in the diagram, we use the
``omit_path_prefixes/1`` option to suppress those path prefixes. Note
that all the paths and URLs must end with a slash for proper handling.
The ``git`` library may be useful to retrieve the commit SHA1 from a
local repo directory.

For both ``path_url_prefixes/3`` and ``omit_path_prefixes/1`` options,
when a path prefix is itself a prefix of another path, the shorter path
must come last to ensure correct links.

See the ``SCRIPT.txt`` file in the tool directory for additional
examples. To avoid retyping such complex goals when updating diagrams,
use the ``doclet`` tool to save and reapply them easily (e.g., by using
the ``make`` tool with the ``documentation`` target).

Creating diagrams for Prolog module applications
------------------------------------------------

Currently limited to SWI-Prolog and YAP Prolog module applications due
to the lack of a comprehensive reflection API in other Prolog systems.

Simply load your Prolog module application and its dependencies and then
use diagram entity, directory, or file predicates. Library diagram
predicates are not supported. See the ``SCRIPT.txt`` file in the tool
directory for some usage examples. Note that support for diagrams with
links to API documentation is quite limited, however, due to the lack of
Prolog standards.

Creating diagrams for plain Prolog files
----------------------------------------

This tool can also be used to create predicate cross-referencing
diagrams for plain Prolog files. For example, if the Prolog file is
named ``code.pl``, simply define an object including its code:

::

   :- object(code).
       :- include('code.pl').
   :- end_object.

Save the object to an e.g. ``code.lgt`` file in the same directory as
the Prolog file and then load it and create the diagram:

::

   | ?- logtalk_load(code),
        xref_diagram::entity(code).

An alternative is to use the ``object_wrapper_hook`` provided by the
``hook_objects`` library:

::

   | ?- logtalk_load(hook_objects(loader)).
   ...

   | ?- logtalk_load(code, [hook(object_wrapper_hook)]),
        xref_diagram::entity(code).

Other notes
-----------

Generating complete diagrams requires that all referenced entities are
loaded. When that is not the case, notably when generating
cross-referencing diagrams, missing entities can result in incomplete
diagrams.

For complex applications, diagrams can often be made simpler and more
readable by omitting external nodes (see the ``externals/1`` option)
and/or using one of the alternatives to ``dot`` provided by Graphviz
depending on the type of the diagram (see the section above on supported
graph languages for more details).

When generating entity predicate call cross-reference diagrams, caller
nodes are not created for auxiliary predicates. For example, if the
``meta_compiler`` library is used to optimize meta-predicate calls, the
diagrams may show predicates that are not apparently called by any other
predicate when the callers are from the optimized meta-predicate goals
(which are called via library generated auxiliary predicates). A
workaround in this case would be creating a dedicated loader file that
doesn't load (and apply) the ``meta_compiler`` library when generating
the diagrams.

When generating diagrams in SVG format, a copy of the ``diagrams.css``
file must exist in any directory used for publishing diagrams using it.
The ``lgt2svg`` scripts also take care of copying this file.

The Graphviz command-line utilities, e.g., ``dot``, are notorious for
random crashes (segmentation faults usually), often requiring retrying
conversions from ``.dot`` files to other formats. A possible workaround
is to repeat the command until it completes without error. See, for
example, the ``lgt2svg.sh`` script.

The conversion by the ``d2`` command-line executable of ``.d2`` files to
``.svg`` files can be quite slow (as of its 0.6.8 version) with the
default ``elk`` layout engine. The ``dagre`` layout engine is much
faster but doesn't support a node referencing itself (notably, a node
representing a metaclass that instantiates itself).

Using the default d2 layout engine (``elk``) works fine with graphs with
a relatively small number of nodes and edges. When that's not the case,
it's a good idea to experiment with other layout engines.
