[Stacked Bar] - Filling in 0 for missing values in the domain.

For stacking purposes only, 0 is provided as a value for stacked bars.  The svg element will not show up.  The 0 is only needed for calculating offsets.
Seems like we might want to factor out the logic of getting the `keyAccessor` and `valueAccessor` -- I see `keyAccessor` used in at least two other places.