Documentation is at http://tcllib.sourceforge.net/doc/graph.html
This part of the struct (data structures) submodule of tcllib provides graph objects, where graph does not mean the plotting of values as a graphic, but mathematical graph structures consisting of nodes and edges.
In the case of this module, directed graphs are provided, i.e. the edges have direction and are called arcs instead.
Applications:
For fun and play see RS's Graph theory in Tcl and the resulting TkMaze.
e4graph is a package by Jacob Levy at http://sourceforge.net/projects/e4graph .
From the home page.... "e4Graph package is a C++ library that provides your program with reliable, efficient and portable persistent storage for graph like data. This package is being developed by Jacob Levy in collaboration with Jean-Claude Wippler, the author of the excellent Metakit database engine."
There is a Tcl binding.
Category Package, see struct / tcllib - Category Mathematics - GOBLIN