Fix extending domainer and update tests.

`Plot.StackedArea`:
* `automaticallyAdjustYScaleOverVisiblePoints` was broken because after switch parent class to `Plot.Area` it used wrong `_normalizeDatasets` and didn't take stacking into account.
* in `_updateYDomainer` baseline has not been added to `domainer`
`Plot.StackedBar`:
* to make consistent with `Plot.StackedArea` it calling `nice()` on `domainer`

Close #1424.