Sync bin widths in grouped histograms

[srosanba]

When a continuous variable is summarized by subgroup with histograms, the bin widths are calculated independently. If the bin widths are not the same, this can be somewhat misleading. Consider making a pooled bin width calculation for subgrouped histograms.

[jwildfire]

I think bins are calculated based on the data distribution for the panel, not on the shared range. Should be relatively forward to update to have the binning based on the shared range.