[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.
can you explain why we want to operate on the `D3.Map<StackedDatum>[]` rather than the `StackedDatum[][]`? i get that its convenient when we are filling in the holes in the data, but why not use maps to fill in the holes but then return arrays for simplicity? also, do maps have an iteration order defined? do we risk getting things in the wrong order somehow? how is that ordering maintained?