I've got MRTG setup on a CentOS 5.3 32-bit machine running in Virtual PC 2007 on a Windows XP SP3 box.
I noticed that the time for my graphs was incorrect, and the time on my CentOS virtual machine was also incorrect. To correct time I used hwclock to set the time to current time. Now my MRTG 5 minute avg graphs are blurry, and some are not returning any data at all. I rebooted the machine, remade the mrtg.cfg and the index.html files, but I still have the problem.