Fix handling of horizontal legendgroup

[antoinerg]

Fixes #1913

See #1913 (comment) for details.

[etpinard]

Can you tell us how legend.tracegroupgap affects legend groups on orientation: 'h' legends now?

[antoinerg]

Can you tell us how legend.tracegroupgap affects legend groups on orientation: 'h' legends now?

@etpinard legend.tracegroupgap changes the horizontal spacing between legend items when the legend is oriented horizontally even in the absence of legend groups. If I set it to 50 in the new mock, we get:

I don't know if this behavior was intentional but the current PR preserves it.

[antoinerg]

I think we need to introduce a new legend attribute to control the spacing between trace items independently of the spacing between grouped items. The new attribute could be named legend.tracegap and would complement the current legend.tracegroupgap.

cc @nicolaskruchten @plotly/plotly_js

[nicolaskruchten]

I don't mind in principle, but can't we get away without it? It seems to me that horizontal legends are just like vertical ones, no?

[antoinerg]

I don't mind in principle, but can't we get away without it?

Yes we can get away without it if we're happy with solely controlling the spacing between groups of items and NOT have control over the spacing between items.

It seems to me that horizontal legends are just like vertical ones, no?

In vertical legends, grouped items are stacked vertically. In horizontal legends, they are also stacked vertically. One may argue that they should be stacked horizontally (as done in the current PR). I personally don't have a preference. If we want to keep them stacked vertically even in horizontal legends, the following community PR #3628 is more appropriate.

cc @nicolaskruchten

[nicolaskruchten]

In horizontal legends, they are also stacked vertically

This is the current behaviour?

[antoinerg]

In horizontal legends, they are also stacked vertically

This is the current behaviour?

Yes it is. This is how horizontal legends with groups are rendered on master:

where items are overflowing on the right (which is essentially what we're trying to fix #1913). Also, legend.tracegroupgap which is supposed to

Sets the amount of vertical space (in px) between legend groups.

actually controls the horizontal spacing between all items if there is at least one group in the legend.

[nicolaskruchten]

Well, if we currently stack them vertically then we can't change it, right? I think PR #3628 makes sense to me: keep vertically-aligned groups and wrap whole groups, honouring tracegroupgap. Any downsides to that @antoinerg ?

[antoinerg]

I think PR #3628 makes sense to me: keep vertically-aligned groups and wrap whole groups, honouring tracegroupgap. Any downsides to that @antoinerg ?

I agree PR #3628 is less of a breaking change. The only downside I can think of is that vertically-aligned groups will take up more space than they would if stacked horizontally.

cc @nicolaskruchten

Closing in favor of PR #3628