feat(plotly): implement subplot-grid-custom (#2881)

## Implementation: `subplot-grid-custom` - plotly

Implements the **plotly** version of `subplot-grid-custom`.

**File:** `plots/subplot-grid-custom/implementations/plotly.py`

**Parent Issue:** #2856

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:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20608469691)*

---------

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Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: github-actions[bot]

plots/subplot-grid-custom/implementations/plotly.py

@@ -0,0 +1,127 @@
+""" pyplots.ai
+subplot-grid-custom: Custom Subplot Grid Layout
+Library: plotly 6.5.0 | Python 3.13.11
+Quality: 92/100 | Created: 2025-12-30
+"""
+
+import numpy as np
+import pandas as pd
+import plotly.graph_objects as go
+from plotly.subplots import make_subplots
+
+
+# Data
+np.random.seed(42)
+
+# Time series data for main plot (spanning 2 columns)
+dates = pd.date_range("2024-01-01", periods=120, freq="D")
+price = 100 + np.cumsum(np.random.randn(120) * 2)
+volume = np.random.randint(50, 200, 120)
+
+# Returns data for histogram
+returns = np.diff(price) / price[:-1] * 100
+
+# Scatter data for correlation plot
+revenue = np.random.uniform(50, 150, 50)
+expenses = revenue * 0.6 + np.random.randn(50) * 15
+
+# Category data for bar chart
+categories = ["Product A", "Product B", "Product C", "Product D"]
+values = [85, 72, 91, 68]
+
+# Create subplot grid: 2 rows × 3 columns
+# Main plot spans 2 columns in top row, small plots in remaining cells
+fig = make_subplots(
+    rows=2,
+    cols=3,
+    specs=[[{"colspan": 2}, None, {}], [{}, {}, {}]],
+    subplot_titles=(
+        "Stock Price (2 Columns)",
+        "Category Performance",
+        "Trading Volume",
+        "Revenue vs Expenses",
+        "Daily Returns Distribution",
+    ),
+    horizontal_spacing=0.08,
+    vertical_spacing=0.15,
+)
+
+# Main plot: Stock price time series (top-left, spans 2 columns)
+fig.add_trace(
+    go.Scatter(
+        x=dates, y=price, mode="lines", line={"color": "#306998", "width": 3}, name="Stock Price", showlegend=False
+    ),
+    row=1,
+    col=1,
+)
+
+# Top-right: Category bar chart
+fig.add_trace(
+    go.Bar(x=categories, y=values, marker_color="#FFD43B", name="Performance", showlegend=False), row=1, col=3
+)
+
+# Bottom-left: Volume bar chart
+fig.add_trace(
+    go.Bar(x=dates[::4], y=volume[::4], marker_color="#306998", opacity=0.7, name="Volume", showlegend=False),
+    row=2,
+    col=1,
+)
+
+# Bottom-center: Scatter plot (revenue vs expenses)
+fig.add_trace(
+    go.Scatter(
+        x=revenue,
+        y=expenses,
+        mode="markers",
+        marker={"color": "#306998", "size": 12, "opacity": 0.7, "line": {"color": "white", "width": 1}},
+        name="Revenue vs Expenses",
+        showlegend=False,
+    ),
+    row=2,
+    col=2,
+)
+
+# Bottom-right: Returns histogram
+fig.add_trace(
+    go.Histogram(x=returns, nbinsx=20, marker_color="#FFD43B", opacity=0.8, name="Returns", showlegend=False),
+    row=2,
+    col=3,
+)
+
+# Update layout
+fig.update_layout(
+    title={"text": "subplot-grid-custom · plotly · pyplots.ai", "font": {"size": 32}, "x": 0.5, "xanchor": "center"},
+    template="plotly_white",
+    showlegend=False,
+    margin={"l": 80, "r": 60, "t": 120, "b": 80},
+)
+
+# Update all axes with proper font sizes
+fig.update_xaxes(tickfont={"size": 14}, title_font={"size": 18})
+fig.update_yaxes(tickfont={"size": 14}, title_font={"size": 18})
+
+# Update specific axes labels
+fig.update_xaxes(title_text="Date", row=1, col=1)
+fig.update_yaxes(title_text="Price ($)", row=1, col=1)
+
+fig.update_xaxes(title_text="Category", row=1, col=3)
+fig.update_yaxes(title_text="Score", row=1, col=3)
+
+fig.update_xaxes(title_text="Date", row=2, col=1)
+fig.update_yaxes(title_text="Volume (K)", row=2, col=1)
+
+fig.update_xaxes(title_text="Revenue ($K)", row=2, col=2)
+fig.update_yaxes(title_text="Expenses ($K)", row=2, col=2)
+
+fig.update_xaxes(title_text="Daily Return (%)", row=2, col=3)
+fig.update_yaxes(title_text="Frequency", row=2, col=3)
+
+# Update annotation font sizes for subplot titles
+for annotation in fig["layout"]["annotations"]:
+    annotation["font"] = {"size": 20}
+
+# Save as PNG
+fig.write_image("plot.png", width=1600, height=900, scale=3)
+
+# Save as HTML (interactive)
+fig.write_html("plot.html", include_plotlyjs=True, full_html=True)

plots/subplot-grid-custom/metadata/plotly.yaml

@@ -0,0 +1,25 @@
+library: plotly
+specification_id: subplot-grid-custom
+created: '2025-12-30T23:54:54Z'
+updated: '2025-12-31T00:01:09Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20608469691
+issue: 2856
+python_version: 3.13.11
+library_version: 6.5.0
+preview_url: https://storage.googleapis.com/pyplots-images/plots/subplot-grid-custom/plotly/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/subplot-grid-custom/plotly/plot_thumb.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/subplot-grid-custom/plotly/plot.html
+quality_score: 92
+review:
+  strengths:
+  - Excellent implementation of custom subplot grid with colspan specification
+  - Clean, well-structured code following KISS principles
+  - Realistic financial dashboard scenario with coherent data story
+  - Proper title format and good use of Python blue/yellow color scheme
+  - All subplots have clear, descriptive titles and axis labels with units
+  - Good use of plotly make_subplots with specs parameter for non-uniform grid
+  weaknesses:
+  - No rowspan demonstration (spec mentions both colspan AND rowspan)
+  - Grid lines not visible (could add subtle grid for better readability)
+  - Scatter plot markers could be slightly larger for the data density (50 points)