This is a part of the struct module in the tcllib distribution.
The doc for this command will joyfully be found at https://core.tcl-lang.org/tcllib/doc/trunk/embedded/md/tcllib/files/modules/struct/graph.md
Use this to create and manipulate directed graph objects.
[anyone have examples of when to use this?]
The companion package struct::graphop provides a number of graph algorithms.