Plotly Sankey Diagram, problem with broken edge

Question

I am using Sankey plot, from plotly, and for some reason I keep getting a weird shape on the edges of my graph, as seen on the image:

I expected that the green line should flow directly to from one edge to another, instead of doing this weird loop.

To reproduce the error, you can run the code below:

import plotly.graph_objects as go

fig = go.Figure(go.Sankey(
    arrangement = "snap",
    node = {
        "label": ["A", "B", "C", "D", "E", "F"],
        "x": [0, 0.1, 0.5, 0.2, 0.3, 0.5],
        "y": [0, 0.5, 0.2, 0.4, 0.2, 0.3],
        'pad':10},  # 10 Pixels
    link = {
        "source": [0, 0, 1, 2, 5, 4, 3, 5],
        "target": [5, 3, 4, 3, 0, 2, 2, 3],
        "value": [1, 2, 1, 1, 1, 1, 1, 2]}))
fig.show()

you should get this result:

The problem is the edge:

Does anybody know how I can fix it?

Thanks.

score 1 · Answer 1 · edited Jan 25 '22 at 20:30

I had the same issue with a Sankey, involving a loop, under Plotly 5.5.0... I tried your test case, and simplified it a bit more to isolate the problem:

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()

It looks like a bug from Plotly (I submitted an issue), but thankfully there's a way around: it doesn't happen if you make sure the 'x' position of the first node is lower than the 'x' position of the last one. You may try again the code above, just swapping 'x' positions:

    "x": [0.2, 0.6],

In short, you just need to sort all your nodes by ascending 'x' position (which means of course modifying link values accordingly, as they are indice-dependant).

This workaround doesn't work if you're not manually specifying node positions-- to me, almost the entire point of plotly is to automatically layout the nodes-- and it is doing that correctly here! it's just drawing the trace wrong. — Brandon Kuczenski, Feb 07 '22 at 20:27
The issue you describe on the plotly repo is different from the issue presented by the OP. Take a look at my answer below. — Brandon Kuczenski, Feb 07 '22 at 21:02

score 0 · Answer 2 · answered Feb 08 '22 at 01:21

Not a solution, unfortunately, but a more complete problem statement. The problem is not limited to graphs when node positions are specified manually. It depends on the graph topology.

I developed the following minimal example containing five links among 4 nodes. if any of the link values below are set to 0, the problem goes away:

import plotly.graph_objects as go
nodes = {'label': ['node 0', 'node 1', 'node 2', 'node 3']}
links = {'source': [0, 1, 2, 2, 3], 'target': [3, 0, 0, 3, 2], 'value': [1, 1, 1, 1, 1]}
go.Figure(go.Sankey(node=nodes, link=links)).show()

I opened another bug: https://github.com/plotly/plotly.py/issues/3587

Plotly Sankey Diagram, problem with broken edge

2 Answers2