feat(pygal): implement polar-line (#2698)

## Implementation: `polar-line` - pygal

Implements the **pygal** version of `polar-line`.

**File:** `plots/polar-line/implementations/pygal.py`

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20596061354)*

---------

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

Author: github-actions[bot]

plots/polar-line/implementations/pygal.py

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+""" pyplots.ai
+polar-line: Polar Line Plot
+Library: pygal 3.1.0 | Python 3.13.11
+Quality: 91/100 | Created: 2025-12-30
+"""
+
+import pygal
+from pygal.style import Style
+
+
+# Custom style for large canvas (4800x2700)
+custom_style = Style(
+    background="white",
+    plot_background="white",
+    foreground="#333",
+    foreground_strong="#333",
+    foreground_subtle="#666",
+    colors=("#306998", "#FFD43B"),
+    title_font_size=72,
+    label_font_size=48,
+    major_label_font_size=42,
+    legend_font_size=48,
+    value_font_size=36,
+    stroke_width=6,
+    opacity=0.9,
+    opacity_hover=1.0,
+)
+
+# Data - Monthly temperature variations (cyclical pattern)
+# 12 months at 30-degree intervals
+months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"]
+
+# Average high temperatures - Northern hemisphere pattern
+avg_high = [5.2, 7.1, 12.3, 16.8, 21.5, 25.2, 28.1, 27.4, 23.2, 17.1, 10.8, 6.3]
+
+# Average low temperatures
+avg_low = [-2.1, -0.8, 3.2, 7.5, 12.1, 16.3, 19.2, 18.5, 14.1, 8.3, 3.1, -0.5]
+
+# Create polar/radar chart (pygal uses Radar for polar-style plots)
+chart = pygal.Radar(
+    width=4800,
+    height=2700,
+    style=custom_style,
+    title="polar-line · pygal · pyplots.ai",
+    show_legend=True,
+    legend_at_bottom=True,
+    legend_box_size=32,
+    dots_size=12,
+    fill=False,
+    show_dots=True,
+    range=(-5, 35),
+    truncate_legend=-1,
+    margin=60,
+)
+
+# X-axis labels (months around the circle)
+chart.x_labels = months
+
+# Add temperature series
+chart.add("Avg High Temp", avg_high)
+chart.add("Avg Low Temp", avg_low)
+
+# Render to PNG and HTML
+chart.render_to_png("plot.png")
+chart.render_to_file("plot.html")

plots/polar-line/metadata/pygal.yaml

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+library: pygal
+specification_id: polar-line
+created: '2025-12-30T12:05:05Z'
+updated: '2025-12-30T12:08:58Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20596061354
+issue: 0
+python_version: 3.13.11
+library_version: 3.1.0
+preview_url: https://storage.googleapis.com/pyplots-images/plots/polar-line/pygal/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/polar-line/pygal/plot_thumb.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/polar-line/pygal/plot.html
+quality_score: 91
+review:
+  strengths:
+  - Excellent choice of seasonal temperature data that perfectly demonstrates cyclical
+    patterns
+  - Clean, professional visual appearance with well-balanced layout
+  - Appropriate use of pygal Radar chart as the closest equivalent to polar-line
+  - Good color contrast between series (blue/yellow) that is colorblind-accessible
+  - Proper title formatting following pyplots.ai conventions
+  - Font sizes appropriately scaled for the 4800x2700 canvas
+  weaknesses:
+  - Radial axis labels lack units (should show °C somewhere)
+  - Could benefit from more distinct styling or pygal-specific features like custom
+    tooltips
+  - Month labels could be slightly larger for better readability at full scale