ArielStv opened a new pull request #4525: Superset issue #4512: multiple 
metrics, group by, opacity, legends for histogram
   _Trying to address issue #4512 and going further than PR #4513._
   This PR gives the ability to have multiple histograms in the same chart. 
Features added:
   - select multiple numeric columns (one histogram per column)
   - add a group by (one histogram per group and column)
   - legends: I used nvd3 legends for consistency with other superset viz using 
   - opacity option: with multiple histograms, it is sometimes nice to have a 
lower opacity for readability.
   There could be a case for using nvd3 for histograms. I looked into it 
quickly and it didn't seem obvious to me how to make it work in a simple way. 
I'm also not a big fan of the `multiBarChart` feature (there is a PR opened for 
this though - #3777 - not sure where it's at).
   Further possible improvements: better way of assigning bin numbers and bar 
widths across groups (right now, all groups are assigned the same number of 
bins, and width is computed accordingly), align bars on the x-axis, assign one 
distinct opacity value per group.
   ![screen shot 2018-03-02 at 5 50 51 
   Of note: the group-by option here is different from the group-by option for 
other viz: there is no aggregation function applied. Instead, the 'raw' data 
for each group is used to plot the histograms.

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