Support trace-specific color sequences in Plotly Express via templates

- 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 #5416

Author: antonymilne

plotly/express/_core.py

@@ -1037,28 +1037,36 @@ def apply_default_cascade(args, constructor=None):
         if args["color_continuous_scale"] is None:
             args["color_continuous_scale"] = sequential.Viridis
 
+    # if color_discrete_sequence not set explicitly or in px.defaults,
+    # see if we can defer to template. Try trace-specific colors first,
+    # then layout.colorway, then set reasonable defaults
     if "color_discrete_sequence" in args:
         if args["color_discrete_sequence"] is None and constructor is not None:
             if constructor == "timeline":
                 trace_type = "bar"
             else:
                 trace_type = constructor().type
             if trace_data_list := getattr(args["template"].data, trace_type, None):
-                collected_colors = [
+                # try marker.color first
+                args["color_discrete_sequence"] = [
                     trace_data.marker.color
                     for trace_data in trace_data_list
                     if hasattr(trace_data, "marker")
                 ]
-                if not collected_colors:
-                    collected_colors = [
+                # fallback to line.color if marker.color not available
+                if not args["color_discrete_sequence"] or not any(args["color_discrete_sequence"]):
+                    args["color_discrete_sequence"] = [
                         trace_data.line.color
                         for trace_data in trace_data_list
                         if hasattr(trace_data, "line")
                     ]
-                if collected_colors:
-                    args["color_discrete_sequence"] = collected_colors
+                # if no trace-specific colors found, reset to None to allow fallback
+                if not args["color_discrete_sequence"] or not any(args["color_discrete_sequence"]):
+                    args["color_discrete_sequence"] = None
+        # fallback to layout.colorway if trace-specific colors not available
         if args["color_discrete_sequence"] is None and args["template"].layout.colorway:
             args["color_discrete_sequence"] = args["template"].layout.colorway
+        # final fallback to default qualitative palette
         if args["color_discrete_sequence"] is None:
             args["color_discrete_sequence"] = qualitative.D3
 

tests/test_optional/test_px/test_px.py

@@ -226,6 +226,87 @@ def test_px_templates(backend):
         pio.templates.default = "plotly"
 
 
+def test_px_templates_trace_specific_colors(backend):
+    import plotly.graph_objects as go
+
+    tips = px.data.tips(return_type=backend)
+
+    # read trace-specific colors from template.data.histogram
+    histogram_template = go.layout.Template()
+    histogram_template.data.histogram = [
+        go.Histogram(marker=dict(color="orange")),
+        go.Histogram(marker=dict(color="purple")),
+    ]
+    fig = px.histogram(tips, x="total_bill", color="sex", template=histogram_template)
+    assert fig.data[0].marker.color == "orange"
+    assert fig.data[1].marker.color == "purple"
+
+    # scatter uses template.data.scatter colors, not histogram colors
+    scatter_template = go.layout.Template()
+    scatter_template.data.scatter = [
+        go.Scatter(marker=dict(color="cyan")),
+        go.Scatter(marker=dict(color="magenta")),
+    ]
+    scatter_template.data.histogram = [
+        go.Histogram(marker=dict(color="orange")),
+    ]
+    fig = px.scatter(tips, x="total_bill", y="tip", color="sex", template=scatter_template)
+    assert fig.data[0].marker.color == "cyan"
+    assert fig.data[1].marker.color == "magenta"
+
+    # histogram still uses histogram colors even when scatter colors exist
+    fig = px.histogram(tips, x="total_bill", color="sex", template=scatter_template)
+    assert fig.data[0].marker.color == "orange"
+
+    # fallback to layout.colorway when trace-specific colors don't exist
+    fig = px.histogram(
+        tips, x="total_bill", color="sex", template=dict(layout_colorway=["yellow", "green"])
+    )
+    assert fig.data[0].marker.color == "yellow"
+    assert fig.data[1].marker.color == "green"
+
+    # timeline special case (maps to bar)
+    timeline_template = go.layout.Template()
+    timeline_template.data.bar = [
+        go.Bar(marker=dict(color="red")),
+        go.Bar(marker=dict(color="blue")),
+    ]
+    timeline_data = {
+        "Task": ["Job A", "Job B"],
+        "Start": ["2009-01-01", "2009-03-05"],
+        "Finish": ["2009-02-28", "2009-04-15"],
+        "Resource": ["Alex", "Max"],
+    }
+    # Use same backend as tips for consistency
+    df_timeline = px.data.tips(return_type=backend)
+    df_timeline = nw.from_native(df_timeline).with_columns(
+        nw.lit("Job A").alias("Task"),
+        nw.lit("2009-01-01").alias("Start"),
+        nw.lit("2009-02-28").alias("Finish"),
+        nw.lit("Alex").alias("Resource"),
+    ).head(1).to_native()
+    # Add second row
+    df_timeline2 = nw.from_native(df_timeline).with_columns(
+        nw.lit("Job B").alias("Task"),
+        nw.lit("2009-03-05").alias("Start"),
+        nw.lit("2009-04-15").alias("Finish"),
+        nw.lit("Max").alias("Resource"),
+    ).head(1).to_native()
+    # Combine - actually, this is getting too complex. Let me just use a simpler approach
+    import pandas as pd
+    df_timeline = pd.DataFrame(timeline_data)
+    fig = px.timeline(
+        df_timeline,
+        x_start="Start",
+        x_end="Finish",
+        y="Task",
+        color="Resource",
+        template=timeline_template,
+    )
+    assert fig.data[0].marker.color == "red"
+    assert fig.data[1].marker.color == "blue"
+
+
 def test_px_defaults():
     px.defaults.labels = dict(x="hey x")
     px.defaults.category_orders = dict(color=["b", "a"])