Graphviz: A Plugin for Pelican

Graphviz is a Pelican plugin that allows the inclusion of Graphviz images using the Markdown markup format. The code for the Graphviz figure is included as a block in the article’s source, and the image is embedded in HTML using the Base64 format.

Installation

This plugin can be installed via:

python -m pip install pelican-graphviz

Graphviz must be installed on the system, otherwise this plugin will be deactivated. Graphviz can be installed on Debian-based systems via:

sudo aptitude install graphviz

For macOS, Graphviz can be installed via Homebrew:

brew install graphviz

Usage

In the Markdown source, the Graphviz code must be inserted as an individual block (i.e., separated from the rest of the material by blank lines), like the following:

..graphviz dot
digraph G {
  graph [rankdir = LR];
  Hello -> World
}

This will insert an image in your article like this:

The block must start with ..graphviz (this is configurable — see below). The word dot in the first line indicates the program that will be run to produce the image. The available programs are: dot, neato, twopi, circo, fdp, sfdp, and patchwork (see the Graphviz documentation for details). The Graphviz code must start in the second line of the block. Notice that newlines are not allowed inside the Graphviz block.

Styling with CSS

The image is generated in HTML with an <img> element inside an HTML element (by default a <div>, but this is configurable — see below). The latter has class graphviz (this is also configurable — see below). A possible CSS styling would be:

div.graphviz  {
    margin: 10px;
}
div.graphviz img {
    display: block;
    padding: 5px;
    margin-left: auto;
    margin-right: auto;
    text-align: center;
    border-style: solid;
    border-width: 1px;
    border-color: rgb(192, 177, 177);
    -webkit-border-radius: 5px;
}

Configuration

The following variables can be set in the Pelican settings file:

• GRAPHVIZ_HTML_ELEMENT: The HTML inside which the generated Graphviz image is inserted (defaults to 'div'; another sensible possibility would be 'span').

• GRAPHVIZ_IMAGE_CLASS: Class of the <div> element including the generated Graphviz image (defaults to 'graphviz').

• GRAPHVIZ_BLOCK_START: Starting tag for the Graphviz block in Markdown (defaults to '..graphviz').

Output Image Format

The format of the embedded image is SVG, and there is currently no way to change it. This format was chosen over others (like PNG) for two reasons. First, the generated SRC string in Base64 seem to be shorter for SVG than for PNG. Second, the image will be available in the browser in a high-quality vectorized format. As a caveat, notice that this choice may prevent display in browsers lacking proper SVG support.

Alternatives

An alternative to this plugin is the Graphviz tag provided by the Liquid Tags plugin, which differs from this plugin in several respects. First, the Liquid Tags version uses the syntax {% graphviz { <program> […] } %}, while this Graphviz plugin uses the Markdown extension syntax ..graphviz <program> […]. Regarding the rendered output, the differences are:

• Both plugins output an element with class="graphviz". However, only the Graphviz plugin allows you to change the class name via a configuration variable (GRAPHVIZ_IMAGE_CLASS).
• Liquid Tags encodes the image as src="data:image/png;base64,[…]", while the Graphviz plugin encodes it as src="data:image/svg+xml;base64,[…]". This has two impacts. First, the size of the HTML code produced by this Graphviz plugin is much smaller. For example, the Base64 string for the Graphviz code digraph graphname {a -> b -> c; b -> d;} is four times smaller. Second, the Liquid Tags version generates a raster image file, whereas this Graphviz plugin produces a vector image that can be zoomed without image quality loss.
• Liquid Tags outputs the Graphviz image inside a <span>, whereas this Graphviz plugin offers a choice for the container element.

To-Do

Contributions that make this plugin work with reStructuredText content would be welcome.

Contributing

Contributions are welcome and much appreciated. Every little bit helps. You can contribute by improving the documentation, adding missing features, and fixing bugs. You can also help out by reviewing and commenting on existing issues.

To start contributing to this plugin, review the Contributing to Pelican documentation, beginning with the Contributing Code section.

License

Copyright © 2015, 2021 Rafael Laboissière rafael@laboissiere.net

This project is licensed under the AGPL 3.0 license.