feat(pygal): implement line-markers (#2619)

## Implementation: `line-markers` - pygal

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

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

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20594559283)*

---------

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

Author: github-actions[bot]

plots/line-markers/implementations/pygal.py

@@ -0,0 +1,74 @@
+""" pyplots.ai
+line-markers: Line Plot with Markers
+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 - Temperature readings over time (sparse experimental data)
+np.random.seed(42)
+x_values = np.arange(0, 12)  # 12 months
+sensor_a = 20 + 5 * np.sin(x_values * np.pi / 6) + np.random.randn(12) * 1.5
+sensor_b = 18 + 4 * np.sin(x_values * np.pi / 6 + 1) + np.random.randn(12) * 1.2
+sensor_c = 22 + 3 * np.sin(x_values * np.pi / 6 + 2) + np.random.randn(12) * 1.0
+
+months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"]
+
+# Custom style for large canvas
+custom_style = Style(
+    background="white",
+    plot_background="white",
+    foreground="#333333",
+    foreground_strong="#333333",
+    foreground_subtle="#666666",
+    colors=("#306998", "#FFD43B", "#E74C3C"),  # Python Blue, Python Yellow, Red
+    title_font_size=72,
+    label_font_size=48,
+    major_label_font_size=42,
+    legend_font_size=42,
+    value_font_size=36,
+    stroke_width=6,
+    opacity=0.9,
+    opacity_hover=1.0,
+    transition="400ms ease-in",
+    font_family="sans-serif",
+)
+
+# Create line chart with markers
+chart = pygal.Line(
+    width=4800,
+    height=2700,
+    style=custom_style,
+    title="line-markers · pygal · pyplots.ai",
+    x_title="Month",
+    y_title="Temperature (°C)",
+    show_dots=True,
+    dots_size=16,
+    stroke_style={"width": 6},
+    show_x_guides=False,
+    show_y_guides=True,
+    legend_at_bottom=False,
+    legend_box_size=32,
+    margin=100,
+    spacing=50,
+    explicit_size=True,
+    truncate_legend=-1,
+    x_label_rotation=0,
+    show_x_labels=True,
+)
+
+# Set x-axis labels
+chart.x_labels = months
+
+# Add data series
+chart.add("Sensor A", sensor_a.tolist())
+chart.add("Sensor B", sensor_b.tolist())
+chart.add("Sensor C", sensor_c.tolist())
+
+# Save as PNG and HTML
+chart.render_to_png("plot.png")
+chart.render_to_file("plot.html")

plots/line-markers/metadata/pygal.yaml

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+library: pygal
+specification_id: line-markers
+created: '2025-12-30T10:38:14Z'
+updated: '2025-12-30T10:45:46Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20594559283
+issue: 0
+python_version: 3.13.11
+library_version: 3.1.0
+preview_url: https://storage.googleapis.com/pyplots-images/plots/line-markers/pygal/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/line-markers/pygal/plot_thumb.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/line-markers/pygal/plot.html
+quality_score: 91
+review:
+  strengths:
+  - Excellent text legibility with well-scaled font sizes for the large canvas
+  - Clear colorblind-safe color palette (blue, yellow, red)
+  - Proper use of pygal marker functionality with show_dots=True and dots_size=16
+  - Realistic temperature sensor scenario with appropriate seasonal variation
+  - Correct title format following pyplots.ai conventions
+  - Good use of custom Style for consistent, professional appearance
+  weaknesses:
+  - Legend placement in upper left creates slight visual imbalance; positioning at
+    bottom would be better
+  - Grid lines only on Y-axis; could add subtle X-guides for complete reference
+  - Marker visibility good but does not use different marker shapes for series as
+    suggested in spec notes (pygal limitation)