Update missing data handling in grouped/stacked bar charts

[mhickle]

Missing bars in period 7 is because the ‘group’ variable is missing for several of those bars. Bars aren’t drawing when the variable specified in mark[].split is missing.

[jwildfire]

Not sure the best way to address this ... might be ok as is ... maybe we should throw a warning when split values are missing?

[jwildfire]

Note that there are 3 points, but only 1 bar in period 7.

[samussiah]

I don't see an issue here, unless we want to add support to associate grouped bars with other marks.