I found this on Hacker News, and it seems to be an old project that no one was talking about which is a shame because it could interesting.

Pikchr was written in plain C by the developers of SQLite. It generates SVG from a simple language. It is lower-level than PlantUML and Mermaid, but it seems to have no dependency.

The user manual has more examples about this tool.

I’ll definitely try to use in future projects where some visual documentation is needed.