[Stacked Bar] - Supporting stacking on negative values

Handles stacking by placing stacking negative values separately below 0 and stacking positive values separately above 0.

EDIT:  Adding quicktest ss below

MOAR EDITS: Modified Brandon's quicktest. (TK)

![screen shot 2014-09-25 at 4 33 24 pm](https://cloud.githubusercontent.com/assets/8551049/4413921/6c0c91ea-450d-11e4-9632-8ec885f76ff6.png)
![screen shot 2014-09-25 at 4 36 34 pm](https://cloud.githubusercontent.com/assets/8551049/4413920/6c05be88-450d-11e4-95ba-b0f5d182b6b1.png)


Closes #813