Interface to the GraphViz graphing tool

This module provides an interface to layout and image generation of
directed and undirected graphs in a variety of formats (PostScript, PNG,
etc.) using the "dot", "neato", "twopi", "circo" and "fdp" programs from
the GraphViz project (http://www.graphviz.org/ or
http://www.research.att.com/sw/tools/graphviz/).