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fix: handle 'node_color' in element transformations for Bokeh and MPL #6678

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fix: handle 'node_color' in element transformations for Bokeh and MPL #6678

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epaaso
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@epaaso epaaso commented Sep 1, 2025

Fixes Issue #3835.

To avoid messing with internal as much as possible, I changed the code such that to be able to use this fix with mpl you have to define the nodes object as a Dataset with the column for color as a vdim. Like in this snippet:

nodes_ds = Dataset(nodes_df, kdims=['label'], vdims=['color'])
sk = hv.Sankey((links_df, nodes_ds), kdims=['source', 'target'], vdims=['value', 'color'])

sk = sk.opts(
        title='Minimal Sankey (matplotlib)',
        labels='label',
        edge_color='color',      # constant or map; here field -> edges (length = edges)
        node_color='color',      # taken from node vdims (length = nodes)
        fig_inches=500/150,
        aspect=350/500,
        bgcolor='white'
    )

The bokeh parameter node_color is for a fixed color, so one has to use node_fill_color instead:

nodes_ds = Dataset(nodes_df, kdims=['label'], vdims=['color'])
sk = hv.Sankey((links_df, nodes_ds), kdims=['source', 'target'], vdims=['value', 'color'])
sk = sk.opts(
        title='Minimal Sankey (bokeh)',
        labels='label',
        edge_color='color',
        node_fill_color='color',
        width=500, height=350,
        bgcolor='white'
    )

There is an automatic assigning of cdim in both the pipeline for these backends and I do not prevent that. I just changed the code so that if node_color is defined, that parameter is given more importance. Maybe a more complete version of this PR would avoid auto-assigning cdim if node_color is given. But as cdim is deprecated, I decided to not waste much time looking for the auto-assign.

@epaaso epaaso mentioned this pull request Sep 1, 2025
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codecov bot commented Sep 2, 2025 •
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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.97%. Comparing base (2dfe271) to head (aeaead6).

Additional details and impacted files 
@@           Coverage Diff           @@
##             main    #6678   +/-   ##
=======================================
  Coverage   88.97%   88.97%           
=======================================
  Files         328      328           
  Lines       70320    70324    +4     
=======================================
+ Hits        62570    62574    +4     
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This pull request has been mentioned on HoloViz Discourse. There might be relevant details there:

https://discourse.holoviz.org/t/node-color-parameter-does-not-work-with-sankey/8245/4

philippjfr
philippjfr reviewed Sep 4, 2025
View reviewed changes
holoviews/plotting/mpl/element.py
@@ -653,6 +653,8 @@ def _apply_transforms(self, element, ranges, style):
elif type(element) is Path:
val = np.concatenate([v.apply(el, ranges=ranges, flat=True)
for el in element.split()])
elif 'node' in k:
val = v.apply(element.nodes)
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I think we still want to provide the ranges here (and in the bokeh case).

philippjfr
philippjfr reviewed Sep 4, 2025
View reviewed changes
holoviews/plotting/bokeh/element.py
@@ -1838,6 +1838,8 @@ def _apply_transforms(self, element, data, ranges, style, group=None):
ds = Dataset({d.name: v for d, v in self.overlay_dims.items()},
list(self.overlay_dims))
val = v.apply(ds, ranges=ranges, flat=True)[0]
elif 'node' in k:
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Ideally this would be implemented by implementing an overridden _element_transform method on GraphPlot. Annoyingly though we don't
pass k through to that method, so for now I guess this is the only thing we can do.

@philippjfr
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Thanks @epaaso, really appreciate the contribution. A simple test in holoviews/tests/plotting/bokeh/test_graph.py would also be greatly appreciated.

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epaaso commented Sep 6, 2025

Hey, of course give me 1-2 days!

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