feat(letsplot): implement line-filled (#2675)

## Implementation: `line-filled` - letsplot

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

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

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20595337286)*

---------

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plots/line-filled/implementations/letsplot.py

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+""" pyplots.ai
+line-filled: Filled Line Plot
+Library: letsplot 4.8.2 | Python 3.13.11
+Quality: 92/100 | Created: 2025-12-30
+"""
+
+import numpy as np
+import pandas as pd
+from lets_plot import *
+
+
+LetsPlot.setup_html()
+
+# 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_effect = 10000 * np.sin(np.pi * months / 6)
+traffic = base_traffic + seasonal_effect + np.random.randn(12) * 3000
+traffic = np.maximum(traffic, 20000)  # Ensure positive values
+
+df = pd.DataFrame({"month": months, "visitors": traffic})
+
+# Create filled line plot
+plot = (
+    ggplot(df, aes(x="month", y="visitors"))
+    + geom_area(fill="#306998", alpha=0.4)
+    + geom_line(color="#306998", size=2)
+    + geom_point(color="#306998", size=5)
+    + labs(x="Month", y="Website Visitors", title="line-filled · letsplot · pyplots.ai")
+    + scale_x_continuous(breaks=list(range(1, 13)))
+    + ggsize(1600, 900)
+    + theme_minimal()
+    + theme(
+        plot_title=element_text(size=24),
+        axis_title=element_text(size=20),
+        axis_text=element_text(size=16),
+        panel_grid_major=element_line(color="#CCCCCC", size=0.5),
+        panel_grid_minor=element_blank(),
+    )
+)
+
+# Save PNG (scale 3x for 4800 × 2700 px)
+ggsave(plot, "plot.png", path=".", scale=3)
+
+# Save HTML for interactivity
+ggsave(plot, "plot.html", path=".")

plots/line-filled/metadata/letsplot.yaml

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+library: letsplot
+specification_id: line-filled
+created: '2025-12-30T11:23:34Z'
+updated: '2025-12-30T11:35:57Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20595337286
+issue: 0
+python_version: 3.13.11
+library_version: 4.8.2
+preview_url: https://storage.googleapis.com/pyplots-images/plots/line-filled/letsplot/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/line-filled/letsplot/plot_thumb.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/line-filled/letsplot/plot.html
+quality_score: 92
+review:
+  strengths:
+  - Excellent use of geom_area + geom_line + geom_point combination for a complete
+    filled line visualization
+  - Perfect title format and well-chosen realistic data context (website traffic)
+  - Clean code structure following KISS principles with proper reproducibility seed
+  - Good visual balance with appropriate fill transparency (0.4) and line visibility
+  weaknesses:
+  - Axis labels lack units (e.g., "Month (2024)" or "Visitors (thousands)")
+  - Data variation could be more pronounced to better showcase the filled area effect