I did the rrdtool upgrade a year ago for testing purposes, but did not keep it, as newer rrdtool versions require several nonstandard graphics/font libraries.
We have kept using the really ancient rrdtool 1.0.0.50 from year 2003/2005 as it is small and suitable for limited flash space.
(I added the cairo and pango libraries manually, but nowadays they exist also in the nonstandard video feed.)
Edit:
I upgraded to 1.7.2, and one example graph is here:
I could try digging up the source changes on weekend, when I am back home.