feat(plotnine): implement line-filled (#2652)

## Implementation: `line-filled` - plotnine

Implements the **plotnine** version of `line-filled`.

**File:** `plots/line-filled/implementations/plotnine.py`

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20595335022)*

---------

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

Author: github-actions[bot]

plots/line-filled/implementations/plotnine.py

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+""" pyplots.ai
+line-filled: Filled Line Plot
+Library: plotnine 0.15.2 | Python 3.13.11
+Quality: 91/100 | Created: 2025-12-30
+"""
+
+import numpy as np
+import pandas as pd
+from plotnine import aes, element_line, element_text, geom_area, geom_line, ggplot, labs, theme, theme_minimal
+
+
+# Data - Monthly website traffic over a year
+np.random.seed(42)
+months = np.arange(1, 13)
+base_traffic = 50000 + np.cumsum(np.random.randn(12) * 5000)
+seasonal = 10000 * np.sin(np.pi * months / 6)
+visitors = base_traffic + seasonal + np.random.randn(12) * 3000
+visitors = np.maximum(visitors, 20000)
+
+df = pd.DataFrame({"Month": months, "Visitors": visitors})
+
+# Plot
+plot = (
+    ggplot(df, aes(x="Month", y="Visitors"))
+    + geom_area(fill="#306998", alpha=0.4)
+    + geom_line(color="#306998", size=2)
+    + labs(x="Month", y="Website Visitors", title="line-filled · plotnine · pyplots.ai")
+    + theme_minimal()
+    + theme(
+        figure_size=(16, 9),
+        text=element_text(size=14),
+        axis_title=element_text(size=20),
+        axis_text=element_text(size=16),
+        plot_title=element_text(size=24),
+        panel_grid_major=element_line(color="#cccccc", size=0.5, alpha=0.3),
+        panel_grid_minor=element_line(alpha=0),
+    )
+)
+
+# Save
+plot.save("plot.png", dpi=300, verbose=False)

plots/line-filled/metadata/plotnine.yaml

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+library: plotnine
+specification_id: line-filled
+created: '2025-12-30T11:21:44Z'
+updated: '2025-12-30T11:24:22Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20595335022
+issue: 0
+python_version: 3.13.11
+library_version: 0.15.2
+preview_url: https://storage.googleapis.com/pyplots-images/plots/line-filled/plotnine/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/line-filled/plotnine/plot_thumb.png
+preview_html: null
+quality_score: 91
+review:
+  strengths:
+  - Excellent layering of geom_area and geom_line to create the filled effect with
+    visible line on top
+  - Well-chosen alpha value (0.4) matches spec recommendation (0.3-0.5)
+  - Clean, professional appearance with consistent Python blue color (#306998)
+  - Good use of theme customization for text sizing and grid styling
+  - Realistic website traffic scenario with appropriate seasonal variation
+  weaknesses:
+  - X-axis displays months as decimals (2.5, 5.0, 7.5) instead of integers (1, 2,
+    3...12) which is confusing for monthly data
+  - Y-axis label could include units (e.g., Website Visitors (thousands) or use K
+    formatting)