I the only gentoo user who uses mrtg with rrdtool. But in any event, here's an ebuild that gives mrtg the rrdtool use flag. I also have an ebuild for routers2.cgi which I'll post separately when I get a chance, in case someone else uses it in the gentoo community.
Created attachment 94892 [details] Latest mrtg ebuild with rrdtool USE flag added.
Created attachment 94894 [details, diff] Patch to mrtg-2.14.5, includes rrdtools and stderr fix.
Created attachment 94896 [details, diff] STDERR patch
I must've forgotten to submit my previous comment. Ah well. MRTG doesn't disconnect from stderr when daemonizing. Included new patch/ebuild to do that.
Patches are applied upstream, so it should be good to go