Weird Sankey cycles when nodes are positionned and not ordered by increasing x values

I had some strange looking arrows in a Sankey (Plotly v 5.5.0), involving loops. The exact same problem was raised here. After trying different things, I found out the problem occured only when using node positions, and if the nodes in a given loop are not ordered by ascending 'x' position. You may give it a try with this very basic example:

import plotly.graph_objects as go
fig = go.Figure(go.Sankey(
    node = {"label": ["First node", "Last node"],
        "x": [0.6, 0.2],
        "y": [0.2, 0.6]},
    link = {
        "source": [0, 1],
        "target": [1, 0],
        "value":  [1, 1]
}))
fig.show()

And then replacing the "x" line by:
"x": [0.2, 0.6],
This looks like a bug, which should be fixed ideally. In the meantime, I strive to first order my nodes by ascending X position, which is rather tedious, since I already have to remove nodes without links (or 0 value links) as explained in other open issues. This involves quite a bit of pre-processing unfortunately.

Inizia con l’esempio minimo di Sankey in Python presente nell’issue e confronta i due ordinamenti x dei nodi. Segui il percorso di rendering di Sankey per identificare perché le posizioni invertite creano frecce di ciclo errate; il lavoro è completato quando entrambi gli ordinamenti eseguono correttamente il rendering del ciclo senza richiedere ai chiamanti di riordinare i nodi.