Split legend when doing multi bucket aggs

[markwalkom]

When you do a viz with a multi bucket agg - eg in a multi layer pie chart - we combine the legend into a single list. This doesn't really make a lot of sense when you look at it, and it's not until you start mousing over the various slices that you can see what is happening.

It'd be great if we could figure out a way to identify the various buckets. Maybe with a slight indentation with a vertical line representing the level (like what you see when creating multiple buckets in the query setup), or a horizontal line between the levels.

[akabbeke]

This would be really nice to have.

[dav3860]

I agree, it would be very nice.

[timroes]

Duplicate of #3402