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:
Category Package, see struct / tcllib