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

[antonymilne]

Summary

Fixes #5416.

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.

[emilykl]

@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.

[antonymilne]

@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]

@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.