feat(altair): implement line-filled (#2663)

## Implementation: `line-filled` - altair

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

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

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20595334243)*

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: github-actions[bot]

plots/line-filled/implementations/altair.py

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+""" pyplots.ai
+line-filled: Filled Line Plot
+Library: altair 6.0.0 | Python 3.13.11
+Quality: 92/100 | Created: 2025-12-30
+"""
+
+import altair as alt
+import numpy as np
+import pandas as pd
+
+
+# Data - Website traffic over a month
+np.random.seed(42)
+days = np.arange(1, 31)
+# Simulate daily visitors with weekly patterns and growth trend
+base = 5000 + days * 100  # Growth trend
+weekly_pattern = 800 * np.sin(2 * np.pi * days / 7)  # Weekly cycle
+noise = np.random.normal(0, 300, len(days))
+visitors = base + weekly_pattern + noise
+visitors = np.maximum(visitors, 1000)  # Ensure positive values
+
+df = pd.DataFrame({"Day": days, "Visitors": visitors})
+
+# Create filled line chart (area chart)
+chart = (
+    alt.Chart(df)
+    .mark_area(
+        line={"color": "#306998", "strokeWidth": 3},
+        color=alt.Gradient(
+            gradient="linear",
+            stops=[
+                alt.GradientStop(color="rgba(48, 105, 152, 0.4)", offset=0),
+                alt.GradientStop(color="rgba(48, 105, 152, 0.1)", offset=1),
+            ],
+            x1=1,
+            x2=1,
+            y1=1,
+            y2=0,
+        ),
+    )
+    .encode(
+        x=alt.X("Day:Q", title="Day of Month", axis=alt.Axis(labelFontSize=18, titleFontSize=22)),
+        y=alt.Y("Visitors:Q", title="Daily Visitors", axis=alt.Axis(labelFontSize=18, titleFontSize=22)),
+        tooltip=["Day:Q", "Visitors:Q"],
+    )
+    .properties(
+        width=1600, height=900, title=alt.Title("line-filled · altair · pyplots.ai", fontSize=28, anchor="middle")
+    )
+    .configure_axis(grid=True, gridOpacity=0.3, gridDash=[3, 3])
+    .configure_view(strokeWidth=0)
+)
+
+# Save as PNG and HTML
+chart.save("plot.png", scale_factor=3.0)
+chart.save("plot.html")

plots/line-filled/metadata/altair.yaml

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+library: altair
+specification_id: line-filled
+created: '2025-12-30T11:22:20Z'
+updated: '2025-12-30T11:30:00Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20595334243
+issue: 0
+python_version: 3.13.11
+library_version: 6.0.0
+preview_url: https://storage.googleapis.com/pyplots-images/plots/line-filled/altair/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/line-filled/altair/plot_thumb.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/line-filled/altair/plot.html
+quality_score: 92
+review:
+  strengths:
+  - Excellent use of Altair gradient feature for the fill, creating a visually appealing
+    fade effect
+  - Line property within mark_area ensures the line is visible on top of the fill
+  - Realistic website traffic data with growth trend and weekly cyclical patterns
+  - Clean, declarative Altair code following library conventions
+  - Proper font sizes for high-resolution output (scale_factor=3.0)
+  - Tooltips included for interactivity
+  weaknesses:
+  - Axis labels lack units (could be "Daily Visitors (count)" or "Day of Month (day)")
+  - The gradient direction could potentially be inverted (darker at line, lighter
+    toward baseline) to better emphasize the data