The mrtg i386 rpm in Powertools shipped with RHL 7 will not run as it was compiled with an older and incompatible version of libpng (version 1.0.8 shipped with RHL7). At least that's what it complains about. I've removed the 1.0.8 libpng and installed a 1.0.5 from RHL 6.2, which works fine.
Also mrtg still ships with "banner" graphics (that go at the bottom of generated pages) which are gifs. Obviously these will not show up as the package now wants pngs.
*** This bug has been marked as a duplicate of 17135 ***