95th Percentile problem

When I enable 95th percentile reporting, most of the graphs in node and interface views (all the ones where the 95th percentile line should appear) cease to function. Where the graph should be is just a little box with a red 'x' in it. I have read the FAQ and checked the ration is correct, restarted the web engine and cleared the graph cache - but the only way to restore the graphs is to turn off 95th percentile. Any ideas? (I am using the demo version at the moment to evaluate the product)