Grap is a language for typesetting graphs specified and first
implemented by Brian Kernighan and Jon Bentley at Bell Labs. It is
an expressive language for describing graphs and incorporating them 
in typeset documents. It is implemented as a preprocessor to
Kernigan's pic language for describing images, so any system that
can use pic can use grap. For sure, TeX and groff can use it.

To be useful, the pic program from the groff package that is included
in Slackware Linux is required.