Superset issue #4512: multiple metrics, group by, opacity, legends for histogram

[ArielStv]

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 nvd3
• 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.

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.

[ArielStv]

@hughhhh @graceguo-supercat @mistercrunch

[ArielStv]

Also, if you guys do decide to merge this, maybe we should change the thumbnail for the histogram viz?

[SpyderRivera]

This is very cool

[rumbin]

I love it!

Any chance to get this PR merged before the next release?

[ArielStv]

@mistercrunch would you mind taking a look at it? I'd be happy to keep working on it if need be.

[mistercrunch]

@ArielStv, I finally took the time to pull the branch and play with this. This is solid, merging!

[SpyderRivera]

[nicholasgcoles]

@mistercrunch was this functionality deprecated at some point? Or does it require a custom configuration of sorts?