feat(pygal): implement area-stacked-percent (#2682)

## Implementation: `area-stacked-percent` - pygal

Implements the **pygal** version of `area-stacked-percent`.

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

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20595341160)*

---------

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

Author: github-actions[bot]

plots/area-stacked-percent/implementations/pygal.py

@@ -0,0 +1,71 @@
+""" pyplots.ai
+area-stacked-percent: 100% Stacked Area Chart
+Library: pygal 3.1.0 | Python 3.13.11
+Quality: 91/100 | Created: 2025-12-30
+"""
+
+import pygal
+from pygal.style import Style
+
+
+# Data - Market share evolution for tech product categories
+years = ["2018", "2019", "2020", "2021", "2022", "2023", "2024"]
+smartphones = [42, 40, 38, 36, 35, 33, 32]
+laptops = [28, 27, 26, 25, 24, 23, 22]
+tablets = [18, 19, 20, 21, 21, 22, 22]
+wearables = [8, 10, 12, 14, 16, 18, 20]
+accessories = [4, 4, 4, 4, 4, 4, 4]
+
+# Custom style for large canvas
+custom_style = Style(
+    background="white",
+    plot_background="white",
+    foreground="#333333",
+    foreground_strong="#333333",
+    foreground_subtle="#666666",
+    colors=("#306998", "#FFD43B", "#3CB371", "#FF6B6B", "#9B59B6"),
+    title_font_size=72,
+    label_font_size=48,
+    major_label_font_size=42,
+    legend_font_size=42,
+    value_font_size=36,
+    opacity=0.85,
+    opacity_hover=0.95,
+    transition="250ms ease-in",
+)
+
+# Create stacked area chart with percentage mode
+chart = pygal.StackedLine(
+    width=4800,
+    height=2700,
+    style=custom_style,
+    title="area-stacked-percent · pygal · pyplots.ai",
+    x_title="Year",
+    y_title="Market Share (%)",
+    fill=True,
+    stack_from_top=False,
+    show_y_guides=True,
+    show_x_guides=False,
+    y_labels_major_every=2,
+    truncate_legend=-1,
+    legend_at_bottom=False,
+    legend_box_size=30,
+    margin=50,
+    spacing=40,
+    dots_size=8,
+    stroke_style={"width": 3},
+)
+
+# Set x-axis labels
+chart.x_labels = years
+
+# Add data series (values already sum to 100% each year)
+chart.add("Smartphones", smartphones)
+chart.add("Laptops", laptops)
+chart.add("Tablets", tablets)
+chart.add("Wearables", wearables)
+chart.add("Accessories", accessories)
+
+# Save as PNG and HTML
+chart.render_to_png("plot.png")
+chart.render_to_file("plot.html")

plots/area-stacked-percent/metadata/pygal.yaml

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+library: pygal
+specification_id: area-stacked-percent
+created: '2025-12-30T11:26:39Z'
+updated: '2025-12-30T11:36:32Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20595341160
+issue: 0
+python_version: 3.13.11
+library_version: 3.1.0
+preview_url: https://storage.googleapis.com/pyplots-images/plots/area-stacked-percent/pygal/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/area-stacked-percent/pygal/plot_thumb.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/area-stacked-percent/pygal/plot.html
+quality_score: 91
+review:
+  strengths:
+  - Correct implementation of 100% stacked area using StackedLine with fill=True
+  - Excellent title format following pyplots.ai convention
+  - Well-chosen, realistic market share scenario with clear temporal trends
+  - Good color palette with five distinguishable colors
+  - Clean KISS code structure with no unnecessary complexity
+  - Properly outputs both PNG and HTML formats
+  - Custom style with appropriate font sizes for large canvas
+  weaknesses:
+  - Y-axis range starts at ~40 instead of 0, which can be misleading for percentage
+    visualization where the full 0-100% context matters
+  - Data is somewhat artificial with perfectly linear trends and static values (Accessories
+    at exactly 4% for all years)