[Stacked Bar] - Supporting stacking on negative values

Handles stacking by placing stacking negative values separately below 0 and stacking positive values separately above 0.

Closes #813 
The semantics come down to the following
  * The value is positive, therefore use the appropriate offset
  * Use the offset that essentially exists (not 0).

I feel the former feels more flexible/less hacky so I'll stick with this approach