stacked group is not rendered correctly when stack group is not adjacent

[adamshe]

when stacked group is set for non adjacent datasets, it seems the group index returned is not correct.
Please see the codepen here:
https://codepen.io/adamshe/pen/bYGRBP

Looking forward to a quick fix. thanks.

[jcopperfield]

The following patch fixes the problem. fix_4897_stacked_group_is_not_rendered_correctly_when_stack_group_is_not_adjacent.patch.txt

[etimberg]

@jcopperfield could you create a PR with that patch?

[adamshe]

I confirmed that this patch works, but all the yaxis id needs to be y-axis-n format