feat(pygal): implement box-horizontal (#2590)

## Implementation: `box-horizontal` - pygal

Implements the **pygal** version of `box-horizontal`.

**File:** `plots/box-horizontal/implementations/pygal.py`

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20593351461)*

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: github-actions[bot]

plots/box-horizontal/implementations/pygal.py

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+""" pyplots.ai
+box-horizontal: Horizontal Box Plot
+Library: pygal 3.1.0 | Python 3.13.11
+Quality: 68/100 | Created: 2025-12-30
+"""
+
+import numpy as np
+import pygal
+from pygal.style import Style
+
+
+# Create HorizontalBox dynamically (pygal doesn't include it natively)
+# This uses pygal's standard mixin pattern for horizontal chart variants
+HorizontalBox = type("HorizontalBox", (pygal.graph.horizontal.HorizontalGraph, pygal.graph.box.Box), {})
+
+# Data - Response times for different service types (in milliseconds)
+np.random.seed(42)
+categories = ["Database Query", "API Gateway", "Authentication", "File Upload", "Image Processing"]
+data = {
+    "Database Query": np.random.lognormal(3.5, 0.6, 80),
+    "API Gateway": np.random.lognormal(3.2, 0.4, 80),
+    "Authentication": np.random.lognormal(3.8, 0.5, 80),
+    "File Upload": np.random.lognormal(4.2, 0.7, 80),
+    "Image Processing": np.random.lognormal(4.5, 0.6, 80),
+}
+
+# Add outliers with more moderate values to avoid excessive whitespace
+data["Database Query"] = np.append(data["Database Query"], [180, 220, 280])
+data["File Upload"] = np.append(data["File Upload"], [350, 420])
+data["Image Processing"] = np.append(data["Image Processing"], [450, 520, 580])
+
+# Custom style using PyPlots colors - scaled for 4800x2700 canvas
+custom_style = Style(
+    background="white",
+    plot_background="white",
+    foreground="#333333",
+    foreground_strong="#333333",
+    foreground_subtle="#666666",
+    colors=("#306998", "#FFD43B", "#4CAF50", "#FF5722", "#9C27B0"),
+    title_font_size=60,
+    label_font_size=40,
+    major_label_font_size=36,
+    legend_font_size=36,
+    value_font_size=32,
+)
+
+# Create horizontal box chart with category labels displayed on y-axis
+chart = HorizontalBox(
+    width=4800,
+    height=2700,
+    style=custom_style,
+    title="box-horizontal · pygal · pyplots.ai",
+    x_title="Response Time (ms)",
+    show_legend=True,
+    legend_at_bottom=True,
+    legend_at_bottom_columns=5,
+    legend_box_size=30,
+    show_y_guides=True,
+    show_x_guides=False,
+    margin=50,
+    margin_left=400,
+    margin_bottom=200,
+    spacing=60,
+    box_mode="tukey",
+    truncate_label=-1,
+    truncate_legend=-1,
+    dots_size=8,
+)
+
+# Set category labels on y-axis (in horizontal mode, x_labels display on the y-axis)
+chart.x_labels = categories
+
+# Add data for each category (legend entries match the x_labels for cross-referencing)
+for category in categories:
+    chart.add(category, data[category].tolist())
+
+# Save outputs
+chart.render_to_file("plot.html")
+chart.render_to_png("plot.png")

plots/box-horizontal/metadata/pygal.yaml

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+library: pygal
+specification_id: box-horizontal
+created: '2025-12-30T09:34:54Z'
+updated: '2025-12-30T10:00:38Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20593351461
+issue: 0
+python_version: 3.13.11
+library_version: 3.1.0
+preview_url: https://storage.googleapis.com/pyplots-images/plots/box-horizontal/pygal/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/box-horizontal/pygal/plot_thumb.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/box-horizontal/pygal/plot.html
+quality_score: 68
+review:
+  strengths:
+  - Creative solution using dynamic class creation to achieve horizontal box plot
+    in pygal
+  - Excellent real-world scenario (service response times)
+  - Good use of pygal custom Style system with appropriate font sizes for 4800x2700
+    canvas
+  - Proper use of Tukey box mode for statistical accuracy
+  - Clean colorblind-safe color palette
+  weaknesses:
+  - Category labels are NOT displayed on the y-axis - they only appear in the legend
+    at bottom, which defeats the purpose of a horizontal box plot
+  - Excessive whitespace on right side - x-axis extends to 3200ms but data only reaches
+    ~1800ms, wasting ~40% of plot width
+  - Outlier dots are very small and difficult to see against the white background
+  - Legend is redundant when it should complement y-axis labels, not replace them