Horizontal legend in columns

[psalmody]

What do you think @n-riesco? This should work?

I've found that legend positioning has issues with the graph I've been testing when layout.width < 780 but I I'm pretty sure it's related to something else, not an issue with this section rendering the legend.

[psalmody]

@n-riesco Would it be best to merge this into PR #1 or as a separate issue on plotly and separate PR? I'm thinking it makes most sense for horizontal margins to not push into the margin.

[n-riesco]

What happens with layout.width < 780 is how legends are meant to work. Plotly tries to position the legend where the user requested and finds that portions of the legend go beyond the visible area. When this happens, Plotly relocates and resizes the legend to make it fit within the plot area. Here's the logic where this happens.

[n-riesco]

I think horizontal legends look much better like this. I would merge this PR into #1 and see what Étienne thinks.