Add bottom-up and right-left orientations for Sankey diagrams

[wf-r]

What

See #7865

Adds two new flow directions to Sankey traces and gives the existing horizontal/vertical modes explicit, direction-named aliases, fully backward compatible.

orientation now accepts:

• h — horizontal, default (unchanged)
• left-right — sources left, flow rightward (synonym of h)
• right-left — sources right, flow leftward (new)
• v — vertical (unchanged)
• top-down — sources top, flow downward (synonym of v)
• bottom-up — sources bottom, flow upward (new)

Why

So far, h is always left→right and v is always top-down. Right-to-left and build-up (bottom-up) flows are common conventions with no equivalent today. This closes both gaps additively, without changing any existing rendering.

Backward compatibility

Purely additive. Existing figures using h/v render identically; the default is unchanged.

[camdecoster]

I'm going to change the target branch from master to our upcoming major version branch, v4.0. We've made some changes in the Sankey code and I'd like to add your changes against that. If you have time, it would be great to get this in for that version. I'll leave you a review tomorrow.

[wf-r]

Thanks for having a look (I have rebased on v4.0 for the moment)

[camdecoster]

This is a great addition and pretty simple to add. Could you please separate out the direction config into a second attribute? I added a suggestion. This will require changes to your code elsewhere, but I don't think it will be too much work. Let me know what you think

[wf-r]

This is a great addition and pretty simple to add. Could you please separate out the direction config into a second attribute? I added a suggestion. This will require changes to your code elsewhere, but I don't think it will be too much work. Let me know what you think

Good idea, I changed it accordingly.

[wf-r]

Ah I see. I have changed where node labels are put in vertical layouts.

Old: on node

New: same as horizontal (behind node flow-wise, last nodes before node)

What's your preference? I have added the horizontal baselines and plot-schema, for vertical baselines I will wait till we discussed label positioning.

[camdecoster]

Let's stick with the existing location. I'll ask for opinions internally. It could be the case that this should be treated as a bug. If that's the case, it can be addressed in a separate PR.

Could you also take a look at sankey/select.js? I think you'll need to make an update there that also handles the reverse direction.

Lastly, link sort and node sort just landed in the v4 branch (in #7873). How do you think that will affect this PR? I think you'll need to merge those changes to make sure things are working.

[camdecoster]

We try to avoid nested ternaries. Could you refactor this to remove that?