Unfortunately, this is one of the pitfall of reporting. The number will look better the more decimal places displayed, but becomes harder to read. There really is no way around it, as the numbers are what they are, and whatever formula you devise, will really be no better than where you are now, except to remove the overage/underage from one group and force the numbers to balance. I don't use charts, so I don't even know if that is possible, even in a regular report.
I would explain that it is a rounding issue and the numbers are indicative of the relative values...the rounding is very small and occurs on all the numbers. 0.6% might really be 0.57%, and if that matters to the end user, ask if they want more decimal places. It might still round(probably will) but it will be a smaller rounding error.
HTH
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