some tools for diagrams in software documentation
- AsciiFlow Editor online web-editor, limited shapes
- Ditaa Generator Java
- Ascidia Generator Python
yUML (ruby on rails) can be hosted yourself, for 49 USD
- Sequence Diagram - js-sequence-diagrams JavaScript, browser, SVG
- Sequence Diagram - flowchart.js JavaScript, browser, SVG
- KangaModeling, dot-like, web-api-renderer, .NET
At this point I think I'm going with yED (free, visio-like, looks powerful) (or another GUI application) and render images that are then embedded in markdown files. That means I don't see the benefit of ASCII based diagrams anymore - they look hard to maintain and only help in source-code-mode (which I'm definitely not optimizing for).