[Hotfix --> master] - Stacked Area with 0 valued datum uses the positive offset value 

Stacked Area is not officially supported on negative values, thus we should be using the positive stack offset value instead of the negative one.

Closes #1155
Not necessarily a `HackHack`, if it's needed to get the abstract class to run correctly for the tests.

A better way to do this is to override `_getDrawer` only on the `renderer` you created -- see https://github.com/palantir/plottable/blob/develop/test/components/baseAxisTests.ts#L70 . Changing the prototype risks damaging other tests.