Google charts (see http://code.google.com/apis/chart/ ) is infinitely better than MRTG but still, uh, still pretty bad.
I wrote a python program that you can find at http://fluffyhome.googlecode.com/svn/trunk/indigo/graphVars.py that goes through the Indigo database (you can see my preivios posts for how temperature data gets into this database), looks for all the changes to the mainTemp variable, and creates an HTML web page that calls googles charts to produce a graph.
An example graph is


