Comment by everybodyknows

There is another, related class of tools, and that is graphical editors of the Unicode box-drawing characters:

https://github.com/lewish/asciiflow

https://github.com/tbanel/uniline/

Or, less ergonomically, the general rectangle-editing commands built into more powerful code editors e.g. Emacs.

With these, simple but fairly pretty box+line+text diagrams can be inlined with your source code comments. This unification may help in the perennial struggle to keep software architecture and source code reality in sync.