coverage: how to see the coverage influx and regressions

[tarasmadan]

Is your feature request related to a problem? Please describe.
I can relatively easy show what coverage was added and what coverage was lost.
For example we have some starting month. It will have the regular coverage.
But all the following periods will show +x% or -x%.
The UI selectors will look like "show coverage influx only", "show coverage regressions only".

Describe the solution you'd like
Coverage diff function is the side-effect of the manager-unique coverage calculation.
This proposal is the step forward to add more use-cases.

[tarasmadan]

Not that easy... Month to month comparison may fail because of the target commit change. But it should be Ok within commit.