feat(altair): implement polar-bar (#2749)

## Implementation: `polar-bar` - altair

Implements the **altair** version of `polar-bar`.

**File:** `plots/polar-bar/implementations/altair.py`

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20601057527)*

---------

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/polar-bar/implementations/altair.py

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+""" pyplots.ai
+polar-bar: Polar Bar Chart (Wind Rose)
+Library: altair 6.0.0 | Python 3.13.11
+Quality: 91/100 | Created: 2025-12-30
+"""
+
+import altair as alt
+import numpy as np
+import pandas as pd
+
+
+# Data - Wind direction frequency data
+np.random.seed(42)
+directions = ["N", "NE", "E", "SE", "S", "SW", "W", "NW"]
+angles = [0, 45, 90, 135, 180, 225, 270, 315]  # degrees from north
+
+# Simulate realistic wind frequency with prevailing westerlies
+base_freq = [12, 8, 10, 6, 9, 18, 22, 16]  # Higher for W, SW, NW
+frequencies = [f + np.random.randint(-2, 3) for f in base_freq]
+
+# Create DataFrame with angle in radians for Altair
+df = pd.DataFrame({"direction": directions, "angle": angles, "frequency": frequencies})
+
+# Calculate start and end angles for each bar (in radians)
+# Altair uses radians, and we need to center each bar on its direction
+bar_width = 40  # degrees
+df["theta_start"] = np.radians(df["angle"] - bar_width / 2)
+df["theta_end"] = np.radians(df["angle"] + bar_width / 2)
+
+# Create the polar bar chart using mark_arc
+base = alt.Chart(df)
+
+# Bars radiating from center using arc marks
+bars = base.mark_arc(stroke="white", strokeWidth=2).encode(
+    theta=alt.Theta("theta_start:Q", scale=alt.Scale(domain=[0, 2 * np.pi])),
+    theta2="theta_end:Q",
+    radius=alt.Radius("frequency:Q", scale=alt.Scale(type="linear", zero=True, rangeMin=0)),
+    color=alt.Color(
+        "frequency:Q",
+        scale=alt.Scale(scheme="blues"),
+        legend=alt.Legend(title="Frequency (days)", titleFontSize=18, labelFontSize=16, orient="right"),
+    ),
+    tooltip=[alt.Tooltip("direction:N", title="Direction"), alt.Tooltip("frequency:Q", title="Frequency (days)")],
+)
+
+# Add direction labels around the perimeter
+max_freq = max(frequencies) * 1.25
+label_df = pd.DataFrame({"direction": directions, "angle_rad": np.radians(angles), "radius": [max_freq] * 8})
+
+# Calculate x, y positions for labels
+label_df["x"] = label_df["radius"] * np.sin(label_df["angle_rad"])
+label_df["y"] = label_df["radius"] * np.cos(label_df["angle_rad"])
+
+labels = (
+    alt.Chart(label_df)
+    .mark_text(fontSize=20, fontWeight="bold", color="#306998")
+    .encode(x=alt.X("x:Q", axis=None), y=alt.Y("y:Q", axis=None), text="direction:N")
+)
+
+# Combine bars and labels
+chart = (
+    alt.layer(bars, labels)
+    .properties(
+        width=900,
+        height=900,
+        title=alt.Title(text="polar-bar · altair · pyplots.ai", fontSize=28, anchor="middle", color="#333333"),
+    )
+    .configure_view(strokeWidth=0)
+    .interactive()
+)
+
+# Save outputs
+chart.save("plot.png", scale_factor=4.5)
+chart.save("plot.html")

plots/polar-bar/metadata/altair.yaml

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+library: altair
+specification_id: polar-bar
+created: '2025-12-30T16:29:55Z'
+updated: '2025-12-30T16:46:59Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20601057527
+issue: 0
+python_version: 3.13.11
+library_version: 6.0.0
+preview_url: https://storage.googleapis.com/pyplots-images/plots/polar-bar/altair/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/polar-bar/altair/plot_thumb.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/polar-bar/altair/plot.html
+quality_score: 91
+review:
+  strengths:
+  - Excellent use of mark_arc for creating polar bar chart wedges with proper theta/theta2
+    angle calculations
+  - Clean direction label positioning using calculated x/y coordinates from polar
+    coordinates
+  - Effective color encoding with sequential blues scheme that enhances data interpretation
+  - Interactive tooltips provide additional data exploration capability
+  - Realistic wind frequency data with prevailing westerlies pattern demonstrates
+    domain knowledge
+  weaknesses:
+  - Legend title shows Frequency (days) but the scale numbers do not include units
+    in tick labels
+  - Scale factor 4.5 produces approximately 4050x4050 pixels which does not match
+    the standard 3600x3600 or 4800x2700 target sizes
+  - Could demonstrate stacked bars for wind speed ranges as mentioned in spec notes