Diagram Generator

This Lua filter is used to create figures from code blocks: images are generated from the code with the help of external programs. The filter processes diagram code for Asymptote, Graphviz, Mermaid, PlantUML, and TikZ.

Usage

The filter modifies the internal document representation; it can be used with many publishing systems that are based on pandoc.

Please make sure to read the section on security if you are going to use this filter with third-party input documents.

Important

This filter makes the generated images available to pandoc, but does not write image files by itself. Use pandoc's --extract-media to write the generated images to disk. Or, when producing HTML, use --embed-resources to incorporate the images in the output file via data URIs.

Plain pandoc

Pass the filter to pandoc via the --lua-filter (or -L) command line option.

pandoc --lua-filter diagram.lua ...

Quarto

Users of Quarto can install this filter as an extension with

quarto install extension pandoc-ext/diagram

and use it by adding diagram to the filters entry in their YAML header.

---
filters:
  - diagram
---

Notes on usage with Quarto

Quarto comes with its own system for diagram generation that can be used for a variety of diagrams. Especially Mermaid diagram generation is much faster with Quarto's built-in diagram handling.

Due to the way in which Quarto handles code blocks, do not add filename attributes to code block attribute lists.

``` {.tikz filename="my-graph"}
% DON'T use the filename attribute on code blocks
...

Instead, use the "comment-pipe" syntax to define the graphic's file name.

``` tikz
%%| filename: my-graph
% This should work ok.
...
```

R Markdown

Use pandoc_args to invoke the filter. See the R Markdown Cookbook for details.

---
output:
  word_document:
    pandoc_args: ['--lua-filter=diagram.lua']
---

Diagram types

The table below lists the supported diagram drawing systems, the class that must be used for the system, and the main executable that the filter calls to generate an image from the code. The environment variables column lists the names of env variables that can be used to specify a specific executable.

System	code block class	executable	env variable
Asymptote	asymptote	asy	ASYMPTOTE_BIN
GraphViz	dot	dot	DOT_BIN
Mermaid	mermaid	mmdc	MERMAID_BIN
PlantUML	plantuml	plantuml	PLANTUML_BIN
TikZ	tikz	pdflatex	PDFLATEX_BIN
cetz	cetz	typst	TYPST_BIN

Other diagram engines

The filter can be extended with local packages; see Configuration below.

Figure options

Options can be given using the syntax pioneered by Quarto:

``` {.dot}
//| label: fig-boring
//| fig-cap: "A boring Graphviz graph."
digraph boring {
  A -> B;
}
```

Configuration

The filter can be configured with the diagram metadata entry.

Currently supported options:

• cache: controls whether the images are cached. If the cache is enabled, then the images are recreated only when their code changes. This option is disabled by default.

• cache-dir: Sets the directory in which the images are cached. The default is to use the pandoc-diagram-filter subdir of the a common caching location. This will be, in the order of preference, the value of the XDG_CACHE_HOME environment variable if it is set, or alternatively %USERPROFILE%\.cache on Windows and $HOME/.cache on all other platforms.

  Caching is disabled if none of the environment variables mentioned above have been defined.

• engine: options for specific engines, e.g. plantuml or mermaid. The options must be nested below the engine name. Allowed settings are either true or false to enable or disable the engine, respectively, or a map of options. The available settings are:

  • mime-type: the output MIME type that should be produced with this engine. This can be used to choose a specific type, or to disable certain output formats. For example, the following disables support for PDF output in PlantUML, which can be useful when the necessary libraries are unavailable on a system:

    diagram:
      engine:
        plantuml:
          mime-type:
            application/pdf: false

  • line_comment_start: the character sequence that starts a line comment; unset or change this to disable or modify the syntax of user options in the diagram code.

  • execpath: the path to the engine's executable. Use this to override the default executable name listed in the table above.

    Use a list to pass additional arguments to the executable. E.g., execpath: ['xelatex' '-halt-on-error'] will use xelatex as the executable and pass -halt-on-error as the first argument.

  • package: if this option is set then the filter will try to require a Lua package with the given name. If the operation is successful, then the result will be used as the compiler for that diagram type.

  • Any other option is passed through to the engine. See the engine-specific settings below.

Engine-specific options

Some engines accept additional options. These options can either be passed globally as part of the respective engine entry, or locally by adding opt-NAME as an attribute to the diagram code block. Global options always override local options for security reasons.

TikZ

The TikZ engine accepts the header-includes and additional-packages options. Both options are added to the intermediary TeX file that is used to produce the output file. The options differ only in how string values are handled, with bare strings in header-includes being escaped and those in additional-packages being treated as TeX code.

While mentioned above, it should be highlighted that the execpath option can be used to select a specific LaTeX engine. The default is pdflatex.

Example:

---
diagram:
  engine:
    tikz:
      execpath: lualatex
      header-includes:
        - '\usepackage{adjustbox}'
        - '\usetikzlibrary{arrows, shapes}'
---

Security

This filter should not be used with untrusted documents, unless local configs prevent the setting of filter options in the metadata: An attacker that can set the execpath for an engine can execute any binary on the system with the user's permissions. It is hence recommended to review any document before using it with this filter to avoid malicious and misuse of the filter.

The security is improved considerably if the diagram metadata field is unset or set to a predefined value before this filter is called, e.g., via another filter or a defaults file.

Here is an example defaults file that configures the filter such that the configs cannot be overwritten by the document.

# file: diagram-filter.yaml
filters: ['diagram.lua']
metadata:
  engine:
    # enable dot/GraphViz and PlantUML with default options
    dot: true
    plantuml: true

    # disable processing of asymptote and Mermaid diagrams
    asymptote: false
    mermaid: false

    # Use LuaLaTeX to compile TikZ, define headers
    tikz:
      execpath: lualatex
      additional-packages: |
        \usepackage{adjustbox}
        \usetikzlibrary{arrows, shapes}

Usage:

pandoc -d diagram-filter ...