Sankey: group nodes

[antoinerg]

Closes part of #3321 by implementing grouping of nodes via a new node.groups attribute. For now, it can be called via Plotly.restyle.

It is based off a recent master and it supersedes PR #3426

Checklist:

• Remove transitions on first render
• Disallow nodes to be in more than one group

[antoinerg]

Here's a Codepen showcasing the new mock introduced in this PR: https://codepen.io/antoinerg/pen/MLxjmB

[etpinard]

1.45.0 is getting delayed (due to my buggy PRs), so let's try get this in 1.45.0!

[etpinard]

@antoinerg let me know if you need help to finish up this PR today.

[etpinard]

@antoinerg can you fix:

?

[etpinard]

Awesome work @antoinerg

Looks like this PR only needs two more interaction tests:

• Sankey: group nodes #3556 (comment)
• Sankey: group nodes #3556 (comment)

and it will be ready to 🚀

[antoinerg]

@etpinard I added tests in ffbafa7 as requested. In the process of writing them, I encountered a show-stopping bug for certain combinations of nodes: for example in mock sankey_energy if you try to group nodes 1 and 3:

Plotly.restyle(gd,{'node.groups':[[[1,3]]]})

CPU usage goes to 100% and the operation never ends. The issue seems limited to d3-sankey: it works fine if the underlying Sankey generator is d3-sankey-circular. One way we could deal with this would be to force the use of d3-sankey-circular when node.groups is defined and not empty. That way, we would not have any breaking changes.

I would still like to investigate why exactly d3-sankey chokes the way it does for some combinations of nodes 🤔

[antoinerg]

The problem mentionned in #3556 (comment) is now fixed in 7094887.

The problem was that the check for circularity was done prior to grouping nodes. Grouping certain combination of nodes (see 1.) can create a circularity and d3-sankey cannot process the resulting graph (hence the infinite loop). By checking for circularity after grouping, we can appropriately switch to d3-sankey-circular.

1. This is obivous: if we have 1 -> 2 -> 3 and we group 1 and 3, we get 2 <-> 3 which now has a circular link. I should have thought of this 🤦‍♂️

[etpinard]

Nicely done 💃