I just wanted to let you know that I am working on a Ruby On Rails plugin for rrdtool.
It's called Colored: http://lennartkoopmann.github.com/colored/
("Colored" is a really nasty combination of "red"/rrd and "colo"/loco/locomotive)
I am already using the plugin for a live application but it is still under development.
Have fun! Merry christmas!
So long Lennart Koopmann