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Hmmm.... if you're looking for an out-of-the-box solution I can't think of one that is used inside XOOPS. I bet there are some very nice graphing scripts and classes out there (I remember Ackbarr showing me one a while back, can't remember or find the link/name tho, sorry). If you're looking for a graphing tool that's already in use within XOOPS (but not ready for your use just yet), check out the xfstats part of the myxoopsforge module.
Herko