feat(pygal): implement histogram-density (#2456)

## Implementation: `histogram-density` - pygal

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

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

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20584330628)*

---------

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

Author: github-actions[bot]

plots/histogram-density/implementations/pygal.py

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+""" pyplots.ai
+histogram-density: Density Histogram
+Library: pygal 3.1.0 | Python 3.13.11
+Quality: 91/100 | Created: 2025-12-29
+"""
+
+import numpy as np
+import pygal
+from pygal.style import Style
+
+
+# Data - Test scores with a realistic bimodal distribution
+np.random.seed(42)
+# Create bimodal distribution (two groups of students)
+scores_group1 = np.random.normal(loc=65, scale=10, size=150)
+scores_group2 = np.random.normal(loc=82, scale=8, size=100)
+scores = np.concatenate([scores_group1, scores_group2])
+scores = np.clip(scores, 0, 100)  # Clip to valid score range
+
+# Calculate histogram bins and density values
+n_bins = 25
+counts, bin_edges = np.histogram(scores, bins=n_bins, density=True)
+bin_centers = (bin_edges[:-1] + bin_edges[1:]) / 2
+
+# Custom style for pyplots
+custom_style = Style(
+    background="white",
+    plot_background="white",
+    foreground="#333333",
+    foreground_strong="#333333",
+    foreground_subtle="#666666",
+    colors=("#306998",),  # Python Blue
+    title_font_size=48,
+    label_font_size=36,
+    major_label_font_size=32,
+    legend_font_size=32,
+    value_font_size=28,
+    stroke_width=0,
+    opacity=0.85,
+)
+
+# Create histogram chart
+chart = pygal.Bar(
+    width=4800,
+    height=2700,
+    style=custom_style,
+    title="histogram-density · pygal · pyplots.ai",
+    x_title="Test Score",
+    y_title="Density (Probability per Unit)",
+    show_legend=False,
+    show_x_guides=False,
+    show_y_guides=True,
+    x_label_rotation=0,
+    margin=60,
+    spacing=1,
+    print_values=False,
+)
+
+# Create bar labels from bin centers (show every 5th label for clarity)
+chart.x_labels = [f"{int(bc)}" if i % 5 == 0 else "" for i, bc in enumerate(bin_centers)]
+
+# Add density histogram data
+chart.add("Density", [{"value": float(c), "label": f"Score: {int(bin_centers[i])}"} for i, c in enumerate(counts)])
+
+# Save outputs
+chart.render_to_file("plot.html")
+chart.render_to_png("plot.png")

plots/histogram-density/metadata/pygal.yaml

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+library: pygal
+specification_id: histogram-density
+created: '2025-12-29T22:47:43Z'
+updated: '2025-12-29T22:54:23Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20584330628
+issue: 0
+python_version: 3.13.11
+library_version: 3.1.0
+preview_url: https://storage.googleapis.com/pyplots-images/plots/histogram-density/pygal/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/histogram-density/pygal/plot_thumb.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/histogram-density/pygal/plot.html
+quality_score: 91
+review:
+  strengths:
+  - Excellent bimodal distribution data that clearly demonstrates density histogram
+    concept
+  - Clean, readable code following KISS principles with proper random seed
+  - Well-formatted title following the required convention
+  - Good use of pygal Style class for comprehensive customization
+  - Appropriate axis labels with descriptive units
+  - Smart x-label spacing (every 5th label) prevents overlap
+  weaknesses:
+  - Spec suggests adding a reference line or theoretical PDF overlay for context,
+    which is not implemented
+  - Could use pygal native Histogram chart type instead of Bar chart for more semantic
+    correctness