More and better legends

[rokotyan]

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Our main legend component (BulletLegend) is simple, it doesn't cover many common use cases. and we don't have a doc page for it!

It's the time to revisit our legends, add new legend types and write docs for them.

[mattp-]

+1 on this. having functionality similar to whats exposed on https://observablehq.com/@d3/color-legend would be super handy