Bar Chart Data Problem in Cognos 10

[cfunkho]

I've upgraded from 8.4 to 10.1 in a test environment, and am now trying to verify that the reports that have been written work the same.  I have a series of reports with bar charts that display incorrect data in Cognos 10.  For example, a data point that correctly shows as 40% in Cognos 8 displays as 50% in Cognos 10 (both in the chart and in the tool tip when you roll over the bar).

There's only one query, and I've run the tabular data for it in both systems, and the tabular data is exactly the same.

The data point in question is a calculation in the form of x / (x +y), and both the Aggregate Function and Rollup Aggregate Function are "Average". 

Any ideas on how to go about troubleshooting this?  Anyone know of any differences in the Average function between versions?

Thanks,
Chris Funkhouser