Graphviz is a neat tool for generating diagrammatic graphs from textual input files. The syntax of the graphing language is very natural and easy to learn. Further more, PlainDoc system integrates full support for Graphviz, and specifically dot(1) tool. You can find more about Graphviz from graphviz.org, including how to download and install this great tool.
However, if you do not wish to draw graphs using Grpahviz, there is no need to install it.