Pikchr is really great. The syntax looks strange at first, but the learning curve is actually low. Within 5 minutes of looking at the pikchr source of some SQLite diagrams, you'll get it. I use it to produce railroad syntax diagrams in the same style as the ones in the SQLite docs. I don't use the Markdown integration; I just generate SVGs using PikchrShow[0] and treat them as images in my HTML-based docs.