[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 
Can we get a test that confirms intended order when series go negative? Say `data2` is stacked "on top of" `data1` such that, when *the values of both series are positive*, `data1`'s bars are drawn [closer to the baseline] than `data2`s bars are. When *the values of both series are negative*, will `data1`'s bars still be drawn [closer to the baseline]?

Essentially, does the stacking order remain unchanged relative to the baseline, or unchanged relative to the page? What happens when only some of the values go negative?