snap.love is a graph-drawing app that gets closest to what I want when I want to draw boxes and arrows. Unlike Graphviz and PlantUML, this tool is for small graphs where you want complete control over layout. Unlike PowerPoint or draw.io, this tool results generates text files that are more amenable to version control. The catch: it's a lot more limited than all these tools; all you can do so far is draw rectangles and edges between them.

Comments gratefully appreciated. Please send them to me by any method of your choice and I'll include them here.