Han-wen,
After trying to get the time to get around to developing a fret-diagrams
context for a couple of years, it's become apparent that I've got too
much other stuff going on to get to it.
I wonder what it might cost to sponsor defelopment of a fret-diagrams
context. The fret-diagrams context would take a \chordmode sequence and
would produce fret diagrams. I envision that fret diagrams would be
stored in a hash-table and indexed with a chord (the result of parsing a
chord in chordmode, I expect).