Fix vertical spacing of legend items in horizontal mode - #6094

💃 This looks great as far as the situation where each group winds up on a separate line, so I think this solves the immediate need.

There is still some behavior that strikes me as odd though when some groups share a line. I would have expected us to end up with a consistent column width so that (a) the nth group on the first line is horizontally aligned with the nth group on subsequent lines, and (b) all the groups on one line are evenly spaced. I guess the current behavior makes sense without legend groups - it's like you're just writing out the legend as a paragraph - but when you have groups involved it creates some weird effects like this:

That I think would look much better with consistent horizontal spacing. Not exactly sure if we should consider this a bug or just a feature request for a new mode that does what I'm suggesting, but can you make an issue where we can follow up on this?