[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.
if the answer is "so it is pluggable so we can do stuff like interpolation, etc", it seems like the function signature here is not correct (it will need to take in more args to figure out the missing value) so id rather just keep it simple with the inlined value and build the pluggable function in a more well-thought-out way