[Fix] Support trace-specific color sequences in Plotly Express via templates by antonymilne · Pull Request #5437 · plotly/plotly.py

antonymilne · 2025-11-27T15:46:44Z

Summary

You can define color sequences in a template on a per-trace basis via template.data.<trace_type>, but these were ignored by Plotly Express. Now they are used.

Changes

Modified apply_default_cascade() to check for trace-specific colors in template.data.<trace_type> before falling back to template.layout.colorway or default qualitative palette
Added test_px_templates_trace_specific_colors

Example Usage

See here for a demonstration of all (I think?) applicable trace types, i.e. those that support marker.color.

import plotly.express as px
import plotly.graph_objects as go

tips = px.data.tips()

# Create a custom template with trace-specific colors
custom_template = go.layout.Template(
    data={
        "histogram": [
            go.Histogram(marker=dict(color="orange")),
            go.Histogram(marker=dict(color="purple")),
        ],
        "scatter": [
            go.Scatter(marker=dict(color="cyan")),
            go.Scatter(marker=dict(color="magenta")),
        ],
    },
    layout=go.Layout(colorway=["yellow", "green"]),
)
template = pio.templates.merge_templates("plotly", custom_template)

# Histogram uses histogram-specific colors
fig1 = px.histogram(tips, x="total_bill", color="sex", template=template)

# Scatter uses scatter-specific colors
fig2 = px.scatter(tips, x="total_bill", y="tip", color="sex", template=template)

# Box plot falls back to layout.colorway
fig3 = px.box(tips, x="day", y="total_bill", color="sex", template=template)

Code PR

I have read through the contributing notes and understand the structure of the package. In particular, if my PR modifies code of plotly.graph_objects, my modifications concern the code generator and not the generated files.
I have added tests or modified existing tests.
For a new feature, I have added documentation examples (please see the doc checklist as well).
I have added a CHANGELOG entry if changing anything substantial.
For a new feature or a change in behavior, I have updated the relevant docstrings in the code.

- Check template.data.<trace_type> for marker.color or line.color before falling back to template.layout.colorway - Handle timeline special case (maps to bar trace type) - Use marker colors first, fall back to line colors if no markers found - Fixes issue plotly#5416

- Modify apply_default_cascade to check template.data.<trace_type> for marker.color or line.color - Fallback to template.layout.colorway if trace-specific colors not found - Add comprehensive tests for trace-specific color sequences - Handle timeline special case (maps to bar trace type) - Follow existing patterns for symbol_sequence and line_dash_sequence Fixes plotly#5416

- Modify apply_default_cascade to read colors from template.data.<trace_type> - Prioritize trace-specific colors over layout.colorway - Add special case for timeline constructor (maps to bar trace type) - Add comprehensive tests for trace-specific color sequences - Test trace type isolation, fallback behavior, and timeline special case

plotly/express/_core.py

tests/test_optional/test_px/test_px.py

emilykl · 2025-12-02T18:07:38Z

@antonymilne This is a great PR, thank you for the contribution!

I left a few small style/clarity comments, once those are resolved I'll approve.

- Make constructor parameter required (was optional with None default) - Refactor trace-specific color extraction to only assign when colors are found - Improve code clarity by checking for non-empty color list before assignment

plotly/express/_core.py

- Remove redundant check that was setting color_discrete_sequence to None - The check is unnecessary since we only assign trace_specific_colors when any() is True - Fallback logic to layout.colorway and qualitative.D3 remains intact

Co-authored-by: Emily KL <4672118+emilykl@users.noreply.github.com>

antonymilne · 2025-12-04T21:29:48Z

@emilykl thanks very much for your review! I've implemented all your suggested changes exactly as you wrote them, with one small simplification where I changed if trace_specific_colors and any(trace_specific_colors) into just if any(trace_specific_colors).

I messed up the commits a bit but you can see all the changes since your last review here: https://github.com/plotly/plotly.py/pull/5437/files/74543f301be86ab121a8069a506ced9aba2a2d53..a3853d2afc113cea2fda3ab98b435e745823abfd.

P.S. if you're in the mood for reviewing, I have another small PR here 😀 #5439

emilykl · 2025-12-04T23:00:30Z

@antonymilne One final change needed -- in plotly/express/_imshow.py, modify the apply_default_cascade call to pass constructor=None. I suspect that will fix the failing tests (although I can't see the failure details because CircleCI seems to be having an outage right now). Then this will be ready to merge!

Thanks for your work on this bugfix, and for the additional PR! I will do my best to review tomorrow or early next week.

antonymilne · 2025-12-05T14:21:55Z

Oops yes sorry, I made the change to imshow.py before but forgot to push it 🤦

There's a couple of failing tests but I'm pretty sure they're not my fault - I was getting similar messages in #5439. Please could you take a look? 🙏 Maybe they're just flaky and need rerunning? I'm not sure.

emilykl

Thanks @antonymilne ! 🚀

emilykl · 2025-12-05T17:48:18Z

@antonymilne Awesome!

You're right, those failing tests are not related to your PR. I'll go ahead and merge.

antonymilne added 5 commits November 26, 2025 17:01

Minor

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antonymilne changed the title ~~Trace-specific colors~~ [Fix] Support trace-specific color sequences in Plotly Express via templates Dec 1, 2025

Add CHANGELOG entry for trace-specific color sequences fix

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antonymilne marked this pull request as ready for review December 1, 2025 12:00

Update changelog

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antonymilne mentioned this pull request Dec 1, 2025

[Bug] Plotly Express does not respect trace-specific color sequences defined in template #5416

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emilykl self-requested a review December 2, 2025 16:43

emilykl self-assigned this Dec 2, 2025

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antonymilne and others added 4 commits December 4, 2025 21:23

Update tests/test_optional/test_px/test_px.py

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Co-authored-by: Emily KL <4672118+emilykl@users.noreply.github.com>

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Merge branch 'main' into main

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emilykl approved these changes Dec 5, 2025

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emilykl merged commit adfdfcd into plotly:main Dec 5, 2025
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emilykl mentioned this pull request Dec 5, 2025

Fix failing docs build by updating call to color_discrete_sequence with new call signature #5442

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[Fix] Support trace-specific color sequences in Plotly Express via templates#5437

[Fix] Support trace-specific color sequences in Plotly Express via templates#5437
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