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I had the same problem; could not run the graphs that were created. Turns out I had (still do) have the file rights screwed up in the rra directory. Once the files are created, go there and do a chmod 777 on them. Then your graphs should show up.
When I figure out how I have this screwed up, I will go back and fix it. I am a complete newbie to cacti; however, this stuff rocks!