Issue with fill in stackgroups

[dmt0]

fill: 'tonexty' (the default when you stack) fills to the other trace even though they’re in different stackgroups. Should fill the first of each stack to zero.

https://codepen.io/dmt0/pen/dqemWo?editors=0010 newPlot @ 0

[alexcjohnson]

Whew, I can make all sorts of fun stuff happen with multiple stackgroups... this example has them out of order in gd.data:

But even if they're in the right order (ie all 'a' group together, then all 'b' group together, then all unstacked) there are still problems right now. Thanks for the report @dmt0!

[alexcjohnson]

Hmm, I think I need to actually reorder the traces internally so each stackgroup (and all groups of unstacked-but-filled-together traces) stays together. Otherwise you can get situations like this:

Where all the fills have the right shape but the layering is ridiculous. You would never do this intentionally, but I can imagine it happening in the editor for example if you make a couple of stacks and then add a new item to an earlier stack. (If you leave the fills semi-transparent the situation is comprehensible... I just made them opaque to highlight the problem)

I first thought of bringing all unstacked traces to the front, but I think the better solution would be to push traces toward the back to join their groups, but otherwise stay in the same order. It would be weird, but I could imagine a user explicitly wanting a line to be drawn behind a stack. Or, perhaps more likely, wanting a single filled-to-zero trace drawn behind a stack.