feat(pygal): implement histogram-stacked (#2627)

## Implementation: `histogram-stacked` - pygal

Implements the **pygal** version of `histogram-stacked`.

**File:** `plots/histogram-stacked/implementations/pygal.py`

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20594562097)*

---------

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

Author: github-actions[bot]

plots/histogram-stacked/implementations/pygal.py

@@ -0,0 +1,73 @@
+""" pyplots.ai
+histogram-stacked: Stacked Histogram
+Library: pygal 3.1.0 | Python 3.13.11
+Quality: 91/100 | Created: 2025-12-30
+"""
+
+import numpy as np
+import pygal
+from pygal.style import Style
+
+
+# Data - Three groups with different distributions (plant growth measurements in cm)
+np.random.seed(42)
+group_a = np.random.normal(25, 5, 150)  # Shade-grown plants
+group_b = np.random.normal(35, 7, 180)  # Partial-sun plants
+group_c = np.random.normal(45, 6, 120)  # Full-sun plants
+
+# Define bin edges
+bin_edges = np.linspace(5, 65, 13)  # 12 bins from 5 to 65 cm
+
+# Compute histogram counts for each group
+counts_a, _ = np.histogram(group_a, bins=bin_edges)
+counts_b, _ = np.histogram(group_b, bins=bin_edges)
+counts_c, _ = np.histogram(group_c, bins=bin_edges)
+
+# Create bin labels (center of each bin)
+bin_labels = [f"{int(bin_edges[i])}-{int(bin_edges[i + 1])}" for i in range(len(bin_edges) - 1)]
+
+# Custom style for pyplots.ai
+custom_style = Style(
+    background="white",
+    plot_background="white",
+    foreground="#333333",
+    foreground_strong="#333333",
+    foreground_subtle="#666666",
+    colors=("#306998", "#FFD43B", "#4CAF50"),  # Python Blue, Yellow, Green
+    title_font_size=60,
+    label_font_size=36,
+    major_label_font_size=32,
+    legend_font_size=36,
+    value_font_size=28,
+    tooltip_font_size=28,
+    stroke_width=2,
+)
+
+# Create stacked bar chart (pygal Bar with stacked data simulates stacked histogram)
+chart = pygal.StackedBar(
+    width=4800,
+    height=2700,
+    style=custom_style,
+    title="histogram-stacked \u00b7 pygal \u00b7 pyplots.ai",
+    x_title="Plant Height (cm)",
+    y_title="Frequency",
+    show_y_guides=True,
+    show_x_guides=False,
+    legend_at_bottom=False,
+    legend_box_size=30,
+    margin=50,
+    spacing=10,
+    truncate_legend=-1,
+)
+
+# Set x-axis labels
+chart.x_labels = bin_labels
+
+# Add stacked data
+chart.add("Shade-grown", counts_a.tolist())
+chart.add("Partial-sun", counts_b.tolist())
+chart.add("Full-sun", counts_c.tolist())
+
+# Save outputs
+chart.render_to_png("plot.png")
+chart.render_to_file("plot.html")

plots/histogram-stacked/metadata/pygal.yaml

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+library: pygal
+specification_id: histogram-stacked
+created: '2025-12-30T10:38:59Z'
+updated: '2025-12-30T10:48:54Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20594562097
+issue: 0
+python_version: 3.13.11
+library_version: 3.1.0
+preview_url: https://storage.googleapis.com/pyplots-images/plots/histogram-stacked/pygal/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/histogram-stacked/pygal/plot_thumb.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/histogram-stacked/pygal/plot.html
+quality_score: 91
+review:
+  strengths:
+  - Excellent implementation of stacked histogram concept using pygal StackedBar
+  - Clean, well-organized code following KISS principles
+  - Realistic plant growth scenario with meaningful group labels
+  - Proper title format and axis labels with units
+  - Good color palette with sufficient contrast between groups
+  - Appropriate use of custom Style for consistent theming
+  weaknesses:
+  - Legend position in top-left corner could be moved to avoid proximity to plot area
+  - Could leverage more pygal-specific features like custom tooltips or value labels