Sounds like a solid idea 👍

I'd vote for adding a len attribute (short for length) similar to what we use current for configuring the span of colorbars and sliders. By default, its maximum value would be 1 corresponding in pixels to:

// for vertical legends
layout.height - (layout.margin.t + layout.margin.b) 

// for horizontal legends
layout.width - (layout.margin.l + layout.margin.r) 

This len attribute would allow users to specify e.g.

layout = {
  len: 1.2
}

which would make the legend span into the bottom margin.

To achieve the effect I showed, I had modified:

-        legendHeightMax = gs.h;
+        legendHeightMax = fullLayout.height - gs.t;

I think gs.h works out to layout.height - (layout.margin.t + layout.margin.b) as you mention.

Would you recommend that (roughly):

x = layout.height - (layout.margin.t + layout.margin.b) 
legendHeightMax = x * opts.layout.len

@rpaskowitz yep, that sounds about right.

Would you mind making a PR so that we can test this solution in detail?

In doing so, we should try to make updatemenus re-use @n-riesco 's brilliant scrollbox module.

+1 for a len attribute. It would also be useful to have this in ggplotly() when trying to layout colorbar(s) with a legend (relatedly, #1244 would be awesome to have)

Hi - this issue has been sitting for a while, so as part of our effort to tidy up our public repositories I'm going to close it. If it's still a concern, we'd be grateful if you could open a new issue (with a short reproducible example if appropriate) so that we can add it to our stack. Cheers - @gvwilson