[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.