feat(altair): implement line-styled (#2621)

## Implementation: `line-styled` - altair

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

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

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20594557223)*

---------

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

Author: github-actions[bot]

plots/line-styled/implementations/altair.py

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+""" pyplots.ai
+line-styled: Styled Line Plot
+Library: altair 6.0.0 | Python 3.13.11
+Quality: 93/100 | Created: 2025-12-30
+"""
+
+import altair as alt
+import numpy as np
+import pandas as pd
+
+
+# Data - Monthly temperature readings from different weather stations
+np.random.seed(42)
+months = np.arange(1, 13)
+month_names = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"]
+
+# Temperature data for 4 weather stations with seasonal patterns
+base_temp = np.array([5, 7, 12, 16, 21, 25, 28, 27, 23, 17, 11, 6])
+station_a = base_temp + np.random.randn(12) * 1.5
+station_b = base_temp - 3 + np.random.randn(12) * 1.5
+station_c = base_temp + 5 + np.random.randn(12) * 1.5
+station_d = base_temp - 6 + np.random.randn(12) * 1.5
+
+# Create long-form DataFrame for Altair
+df = pd.DataFrame(
+    {
+        "Month": month_names * 4,
+        "MonthNum": list(months) * 4,
+        "Temperature": np.concatenate([station_a, station_b, station_c, station_d]),
+        "Station": (["Coastal"] * 12 + ["Mountain"] * 12 + ["Valley"] * 12 + ["Highland"] * 12),
+    }
+)
+
+# Define line styles (strokeDash) for each station
+line_styles = {
+    "Coastal": [0],  # solid
+    "Mountain": [8, 4],  # dashed
+    "Valley": [2, 2],  # dotted
+    "Highland": [8, 4, 2, 4],  # dash-dot
+}
+
+# Colors - using Python palette and accessible colors
+station_colors = {
+    "Coastal": "#306998",  # Python Blue
+    "Mountain": "#FFD43B",  # Python Yellow
+    "Valley": "#48A9A6",  # Teal
+    "Highland": "#E76F51",  # Coral
+}
+
+# Create the chart
+chart = (
+    alt.Chart(df)
+    .mark_line(
+        strokeWidth=4  # Thicker lines for visibility
+    )
+    .encode(
+        x=alt.X(
+            "MonthNum:O",
+            title="Month",
+            axis=alt.Axis(
+                labelFontSize=18,
+                titleFontSize=22,
+                values=list(months),
+                labelExpr="['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'][datum.value - 1]",
+            ),
+        ),
+        y=alt.Y(
+            "Temperature:Q",
+            title="Temperature (\u00b0C)",
+            scale=alt.Scale(domain=[-5, 40]),
+            axis=alt.Axis(labelFontSize=18, titleFontSize=22),
+        ),
+        color=alt.Color(
+            "Station:N",
+            scale=alt.Scale(domain=list(station_colors.keys()), range=list(station_colors.values())),
+            legend=alt.Legend(
+                title="Weather Station", titleFontSize=20, labelFontSize=18, symbolStrokeWidth=4, symbolSize=300
+            ),
+        ),
+        strokeDash=alt.StrokeDash(
+            "Station:N", scale=alt.Scale(domain=list(line_styles.keys()), range=list(line_styles.values())), legend=None
+        ),  # Combine with color legend
+    )
+    .properties(
+        width=1600,
+        height=900,
+        title=alt.Title(text="line-styled \u00b7 altair \u00b7 pyplots.ai", fontSize=28, anchor="middle"),
+    )
+    .configure_axis(grid=True, gridOpacity=0.3, gridDash=[2, 2])
+    .configure_view(strokeWidth=0)
+)
+
+# Save as PNG (scale_factor=3 gives 4800x2700)
+chart.save("plot.png", scale_factor=3.0)
+
+# Save as HTML for interactivity
+chart.save("plot.html")

plots/line-styled/metadata/altair.yaml

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+library: altair
+specification_id: line-styled
+created: '2025-12-30T10:38:24Z'
+updated: '2025-12-30T10:46:24Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20594557223
+issue: 0
+python_version: 3.13.11
+library_version: 6.0.0
+preview_url: https://storage.googleapis.com/pyplots-images/plots/line-styled/altair/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/line-styled/altair/plot_thumb.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/line-styled/altair/plot.html
+quality_score: 93
+review:
+  strengths:
+  - Excellent use of all four standard line styles (solid, dashed, dotted, dash-dot)
+    clearly distinguishable
+  - Beautiful color palette with Python Blue and accessible complementary colors
+  - Professional typography with well-sized fonts for 4800x2700 output
+  - Realistic weather station temperature scenario with seasonal patterns
+  - Clean, KISS-compliant code structure following Altair best practices
+  weaknesses:
+  - Legend shows only color swatches, not the actual line styles (Altair limitation
+    - strokeDash legend combined with color shows color only)
+  - No interactive features (tooltips, hover) which are Altair distinctive strength