Graphviz: Open-source graph visualization software
Here are some cool things I've done recently with graphviz:1. Draw "sketch" style graphs using https://sketchviz.com/new2. Draw graphs of memory usage in golang programs (using pprof): https://graphviz.org/Gallery/directed/pprof.html3. Embed graphviz into Sphinx docs (shameless self-promotion:  https://docs.robusta.dev/master/catalog/triggers/index.html#...)