Add network graphs

It would be lovely to be able to plot simple network graphs.
https://en.wikipedia.org/wiki/Network_theory
Chart size may be an issue but for small plots nodes may be constructed using icons or small colored unicode characters that may be explained in a legend...

The issue names no files, tests, or entry points. Start by reading the linked network-theory reference and inspecting the existing terminal chart functionality; done means small network graphs can be plotted with a workable node representation, chart sizing, and any needed legend.

索引モデルが issue の本文から書いたものです。