A plugin to provide GraphViz functionality for WordPress sites.
GraphViz is a way of generating visualisations of structural relationships between objects. Almost any kind of diagram where something connects to something else can be drawn and automatically laid out using the DOT language.
[wp_graphviz] shortcode. Various uses are explained in the "How to use" section.
There are however several other introductions to Graphviz and DOT, including an excerpt on the O'Reilly LinuxDevCenter.com site. Another approach would be to look at the examples in the Graphviz gallery.
If in doubt, find an example and experiment with it.