feat(plotnine): implement polar-bar (#2766)

## Implementation: `polar-bar` - plotnine

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

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

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20601058272)*

---------

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

Author: github-actions[bot]

plots/polar-bar/implementations/plotnine.py

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+""" pyplots.ai
+polar-bar: Polar Bar Chart (Wind Rose)
+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,
+    coord_fixed,
+    element_blank,
+    element_text,
+    geom_path,
+    geom_polygon,
+    geom_segment,
+    geom_text,
+    ggplot,
+    labs,
+    scale_fill_manual,
+    scale_x_continuous,
+    scale_y_continuous,
+    theme,
+)
+
+
+# Data - Wind direction frequencies (8 compass directions)
+np.random.seed(42)
+directions = ["N", "NE", "E", "SE", "S", "SW", "W", "NW"]
+direction_angles = [0, 45, 90, 135, 180, 225, 270, 315]  # Degrees from North, clockwise
+frequencies = [15, 8, 12, 5, 18, 22, 10, 7]
+
+# Create polygons for each bar (wedge shape)
+# Each bar spans ±22.5 degrees (45 degrees total width for 8 directions)
+bar_half_width = 18  # degrees, slightly less than 22.5 for visual gap
+bar_rows = []
+bar_id = 0
+
+for direction, angle, freq in zip(directions, direction_angles, frequencies, strict=True):
+    # Create wedge polygon points (from center outward)
+    # Start angle and end angle
+    start_angle = angle - bar_half_width
+    end_angle = angle + bar_half_width
+
+    # Create polygon vertices: center -> arc at radius -> back to center
+    # Convert to math convention: 0° at East, CCW positive
+    # Polar convention: 0° at North (top), CW positive
+
+    # Center point
+    points = [(0, 0)]
+
+    # Arc points at the outer radius
+    arc_angles = np.linspace(start_angle, end_angle, 10)
+    for a in arc_angles:
+        # Convert from compass (N=0, CW) to math (E=0, CCW)
+        theta = np.radians(90 - a)
+        x = freq * np.cos(theta)
+        y = freq * np.sin(theta)
+        points.append((x, y))
+
+    # Close back to center
+    points.append((0, 0))
+
+    # Add all points to dataframe
+    for i, (x, y) in enumerate(points):
+        bar_rows.append({"x": x, "y": y, "direction": direction, "bar_id": bar_id, "order": i})
+
+    bar_id += 1
+
+bar_df = pd.DataFrame(bar_rows)
+
+# Create circular gridlines (concentric circles at magnitude intervals)
+grid_rows = []
+grid_angles = np.linspace(0, 2 * np.pi, 101)
+max_radius = max(frequencies) + 5
+grid_radii = [5, 10, 15, 20, 25]
+
+for radius in grid_radii:
+    if radius <= max_radius:
+        for angle in grid_angles:
+            grid_rows.append({"x": radius * np.cos(angle), "y": radius * np.sin(angle), "radius": radius})
+
+grid_df = pd.DataFrame(grid_rows)
+
+# Create radial spokes (8 compass directions)
+spoke_rows = []
+for deg in direction_angles:
+    angle = np.radians(90 - deg)
+    spoke_rows.append(
+        {"x1": 0, "y1": 0, "x2": (max_radius + 2) * np.cos(angle), "y2": (max_radius + 2) * np.sin(angle)}
+    )
+
+spoke_df = pd.DataFrame(spoke_rows)
+
+# Create compass direction labels
+label_rows = []
+label_radius = max_radius + 5
+for deg, lbl in zip(direction_angles, directions, strict=True):
+    angle = np.radians(90 - deg)
+    label_rows.append({"label": lbl, "x": label_radius * np.cos(angle), "y": label_radius * np.sin(angle)})
+
+label_df = pd.DataFrame(label_rows)
+
+# Create radius labels (frequency values) - positioned along NNE axis
+radius_labels = []
+label_angle = np.radians(90 - 22.5)  # NNE direction
+for r in [5, 10, 15, 20]:
+    if r <= max_radius:
+        radius_labels.append({"label": f"{r}", "x": r * np.cos(label_angle) + 1.5, "y": r * np.sin(label_angle)})
+
+radius_label_df = pd.DataFrame(radius_labels)
+
+# Color palette - alternating Python Blue and Yellow
+colors = {
+    "N": "#306998",
+    "NE": "#FFD43B",
+    "E": "#306998",
+    "SE": "#FFD43B",
+    "S": "#306998",
+    "SW": "#FFD43B",
+    "W": "#306998",
+    "NW": "#FFD43B",
+}
+
+# Plot
+plot = (
+    ggplot()
+    # Circular gridlines (frequency circles)
+    + geom_path(
+        aes(x="x", y="y", group="radius"), data=grid_df, color="#CCCCCC", size=0.5, alpha=0.5, linetype="dashed"
+    )
+    # Radial spokes (direction lines)
+    + geom_segment(aes(x="x1", y="y1", xend="x2", yend="y2"), data=spoke_df, color="#CCCCCC", size=0.5, alpha=0.5)
+    # Bar wedges (wind rose bars)
+    + geom_polygon(
+        aes(x="x", y="y", group="bar_id", fill="direction"),
+        data=bar_df,
+        color="#333333",
+        size=0.5,
+        alpha=0.85,
+        show_legend=False,
+    )
+    # Compass direction labels
+    + geom_text(aes(x="x", y="y", label="label"), data=label_df, size=16, color="#333333", fontweight="bold")
+    # Frequency labels
+    + geom_text(aes(x="x", y="y", label="label"), data=radius_label_df, size=10, color="#666666", ha="left")
+    # Custom colors for directions
+    + scale_fill_manual(values=colors)
+    # Equal coordinate system for proper circles
+    + coord_fixed(ratio=1)
+    # Axis scaling with padding
+    + scale_x_continuous(limits=(-35, 35))
+    + scale_y_continuous(limits=(-35, 35))
+    # Title
+    + labs(title="Wind Direction Frequency · polar-bar · plotnine · pyplots.ai")
+    # Clean polar-style theme
+    + theme(
+        figure_size=(12, 12),
+        plot_title=element_text(size=24, ha="center"),
+        axis_title=element_blank(),
+        axis_text=element_blank(),
+        axis_ticks=element_blank(),
+        axis_line=element_blank(),
+        panel_grid_major=element_blank(),
+        panel_grid_minor=element_blank(),
+        panel_background=element_blank(),
+        plot_background=element_blank(),
+    )
+)
+
+# Save
+plot.save("plot.png", dpi=300, verbose=False)

plots/polar-bar/metadata/plotnine.yaml

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+library: plotnine
+specification_id: polar-bar
+created: '2025-12-30T16:31:23Z'
+updated: '2025-12-30T16:43:38Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20601058272
+issue: 0
+python_version: 3.13.11
+library_version: 0.15.2
+preview_url: https://storage.googleapis.com/pyplots-images/plots/polar-bar/plotnine/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/polar-bar/plotnine/plot_thumb.png
+preview_html: null
+quality_score: 91
+review:
+  strengths:
+  - Excellent manual construction of polar coordinate system using geom_polygon wedges
+  - 'Clean layered composition: gridlines → spokes → wedges → labels'
+  - Proper compass angle convention (N=0°, clockwise) correctly converted to Cartesian
+    coordinates
+  - Appropriate use of 1:1 aspect ratio for circular visualization
+  - Professional appearance with subtle gridlines and balanced colors
+  weaknesses:
+  - Figure size is 12x12 instead of the recommended 16x9 landscape (though square
+    is acceptable for polar plots)
+  - Missing frequency unit in title or labels (e.g., days or %)