Filtering on Diff Report Charts

[richardhsu]

This is to refer to the enhancements from #294 and splitting the feature enhancement from the bug fix.

Basically we should have filtering on the chart selection so it makes it easier to find the metrics we need. This requires reworking a bit of the logic from the original chart filtering. The chart filtering utilizes a global variable to do the filtering so we'd have to rework it for the diff report.