Fix plotly#5381: wrong x/y positions in Sankey with isolated nodes

[FredericoMendes10]

Link to issue

Closes #5381

Description of change

In Sankey diagrams with arrangement='fixed', Plotly.js builds an internal
node array excluding isolated nodes. The fixed-arrangement loop maps position
array indices directly to this internal array assuming a 1:1 correspondence.
When isolated nodes appear before connected nodes in the original array,
positions shift and are applied to the wrong nodes.

Fix: in plotly/io/_kaleido.py, reorder per-node arrays before rendering so
that isolated nodes appear last, aligning the Plotly.js internal array with
the x/y position arrays for all connected nodes.

Demo

https://youtu.be/y5ueoOPuKto

Testing strategy

Added a new test test_sankey_isolated_nodes_fixed_arrangement that creates a
Sankey diagram with isolated nodes before connected nodes and verifies the
rendered x/y positions are correct.

Additional information (optional)

N/A

Guidelines

• I have reviewed the pull request guidelines and the Code of Conduct and confirm that this PR follows them.
• I have added an entry to the changelog if needed (not required for documentation PRs).

[emilykl]

Hi @FredericoMendes10, thanks for the contribution!

Thanks for looking into this issue. This is really helpful information:

1. Isolated nodes: Plotly.js excludes nodes with no valid links from its
   internal b.nodes array, then applies i.node.x[p] to b.nodes[p]
   assuming a 1:1 index correspondence. Isolated nodes before connected nodes
   shift this correspondence, misapplying positions to the wrong nodes.
   2. Phantom nodes: out-of-bounds source/target indices cause Plotly.js to
   compute _ = max(sources+targets) + 1, inflating b.nodes with
   unlabelled entries that further break the index alignment.

If we make this change only in _kaleido.py as proposed, that will only fix the issue for static images, not interactive plots. Based on this info it seems this is really an underlying issue in plotly.js, so the issue should really be fixed directly in that library.

Would you be interested in working on a fix in plotly.js?

In any case, could you also post a comment in #5381 documenting what you've learned?

Thanks again for looking into this issue, it's much appreciated!