feat(plotnine): implement parliament-basic (#2519)

## Implementation: `parliament-basic` - plotnine

Implements the **plotnine** version of `parliament-basic`.

**File:** `plots/parliament-basic/implementations/plotnine.py`

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20585401490)*

---------

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

Author: github-actions[bot]

plots/parliament-basic/implementations/plotnine.py

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+""" pyplots.ai
+parliament-basic: Parliament Seat Chart
+Library: plotnine 0.15.2 | Python 3.13.11
+Quality: 92/100 | Created: 2025-12-30
+"""
+
+import numpy as np
+import pandas as pd
+from plotnine import (
+    aes,
+    coord_fixed,
+    element_rect,
+    element_text,
+    geom_point,
+    ggplot,
+    guide_legend,
+    guides,
+    labs,
+    scale_color_manual,
+    theme,
+    theme_void,
+)
+
+
+# Data - fictional parliament with neutral party names
+parties = [
+    {"party": "Progressive Alliance", "seats": 85, "color": "#306998"},
+    {"party": "Center Coalition", "seats": 72, "color": "#FFD43B"},
+    {"party": "Conservative Union", "seats": 68, "color": "#4ECDC4"},
+    {"party": "Green Future", "seats": 42, "color": "#2ECC71"},
+    {"party": "Liberal Democrats", "seats": 35, "color": "#9B59B6"},
+    {"party": "Independent Group", "seats": 18, "color": "#95A5A6"},
+]
+
+total_seats = sum(p["seats"] for p in parties)
+
+# Calculate seat positions in semicircular arcs
+# Determine optimal row configuration
+n_rows = 8
+inner_radius = 2.0
+row_spacing = 1.0
+
+# Calculate seats per row (more seats in outer rows)
+seats_per_row = []
+for i in range(n_rows):
+    radius = inner_radius + i * row_spacing
+    # Seats roughly proportional to arc length
+    row_seats = int(np.ceil(radius * 3.5))
+    seats_per_row.append(row_seats)
+
+# Adjust to match total seats
+total_calc = sum(seats_per_row)
+scale = total_seats / total_calc
+seats_per_row = [max(3, int(round(s * scale))) for s in seats_per_row]
+
+# Fine-tune to exact total
+diff = total_seats - sum(seats_per_row)
+for i in range(abs(diff)):
+    idx = i % n_rows
+    if diff > 0:
+        seats_per_row[n_rows - 1 - idx] += 1
+    else:
+        seats_per_row[n_rows - 1 - idx] -= 1
+
+# Generate all seat positions with angles
+all_seats = []
+for row_idx, num_seats in enumerate(seats_per_row):
+    radius = inner_radius + row_idx * row_spacing
+    angles = np.linspace(np.pi * 0.95, np.pi * 0.05, num_seats)
+    for angle in angles:
+        all_seats.append({"angle": angle, "radius": radius, "x": radius * np.cos(angle), "y": radius * np.sin(angle)})
+
+# Sort seats by angle (left to right) for party assignment
+all_seats.sort(key=lambda s: -s["angle"])
+
+# Assign parties to seats (left to right across the hemicycle)
+seat_data = []
+cumulative = 0
+for seat in all_seats:
+    # Find which party this seat belongs to
+    running_total = 0
+    for party in parties:
+        running_total += party["seats"]
+        if cumulative < running_total:
+            seat_data.append({"x": seat["x"], "y": seat["y"], "party": party["party"]})
+            break
+    cumulative += 1
+
+df = pd.DataFrame(seat_data)
+
+# Create color mapping and legend labels
+party_colors = {p["party"]: p["color"] for p in parties}
+party_order = [p["party"] for p in parties]
+seat_counts = {p["party"]: p["seats"] for p in parties}
+legend_labels = {p: f"{p} ({seat_counts[p]})" for p in party_order}
+
+# Convert party to categorical with order
+df["party"] = pd.Categorical(df["party"], categories=party_order, ordered=True)
+
+# Create plot
+plot = (
+    ggplot(df, aes(x="x", y="y", color="party"))
+    + geom_point(size=5, alpha=0.95)
+    + scale_color_manual(values=party_colors, labels=lambda x: [legend_labels.get(p, p) for p in x])
+    + labs(title="parliament-basic · plotnine · pyplots.ai", color="")
+    + coord_fixed(ratio=1)
+    + theme_void()
+    + theme(
+        figure_size=(16, 9),
+        plot_title=element_text(size=26, ha="center", weight="bold", ma="center"),
+        legend_title=element_text(size=16),
+        legend_text=element_text(size=13),
+        legend_position="bottom",
+        legend_direction="horizontal",
+        legend_key_size=20,
+        plot_background=element_rect(fill="white", color="white"),
+        panel_background=element_rect(fill="white", color="white"),
+        plot_margin=0.1,
+    )
+    + guides(color=guide_legend(nrow=2, override_aes={"size": 8}))
+)
+
+# Save
+plot.save("plot.png", dpi=300, width=16, height=9, verbose=False)

plots/parliament-basic/metadata/plotnine.yaml

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+library: plotnine
+specification_id: parliament-basic
+created: '2025-12-30T00:03:48Z'
+updated: '2025-12-30T00:09:26Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20585401490
+issue: 0
+python_version: 3.13.11
+library_version: 0.15.2
+preview_url: https://storage.googleapis.com/pyplots-images/plots/parliament-basic/plotnine/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/parliament-basic/plotnine/plot_thumb.png
+preview_html: null
+quality_score: 92
+review:
+  strengths:
+  - Excellent semicircular seat arrangement algorithm with proper left-to-right party
+    assignment
+  - Clean legend with seat counts using lambda for label formatting
+  - Good use of plotnine grammar of graphics with theme_void and coord_fixed
+  - Proper categorical ordering for consistent legend display
+  - Uses neutral fictional party names avoiding political controversy
+  weaknesses:
+  - Seat markers could be slightly larger for better visibility at full resolution
+  - Does not include optional majority threshold line mentioned in spec