Stacked bar charts are invalid when showing percent change from another model run

[nickrsan]

The application calculates the percent change between a comparison model run and the other model runs by differencing the raw values then dividing. Then, if a user enables the chart stacking, it sums up the percents. For all non-percent values, summing the model run differences is fine, but for percents it produces a meaningless result.

We need to either swap the order so that we sum the differences and then divide by the total rather than getting the percents and summing them, or we need to disable stacking when we are in percent change mode (which has edge cases to detect similar to enabling the baseline run).