Use a stacked y axis for Issue Trend chart

The modified visualization introduced in commit fa96f4399fc9c2f84bba815fcc230585087af525
was confusing, due to the Still open dataset being the delta of
Reported - Resolved.

We now display the areas under the resolved and still open datasets
as stacked data. Since the sum of the 2 is equal to the reported set,
its line overlaps and we avoid confusing display.

Fixes 0034847