feat(altair): implement parliament-basic (#2514)

## Implementation: `parliament-basic` - altair

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

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

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20585401343)*

---------

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/parliament-basic/implementations/altair.py

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+""" pyplots.ai
+parliament-basic: Parliament Seat Chart
+Library: altair 6.0.0 | Python 3.13.11
+Quality: 90/100 | Created: 2025-12-30
+"""
+
+import altair as alt
+import numpy as np
+import pandas as pd
+
+
+np.random.seed(42)
+
+# Data: Fictional parliament with 300 seats
+parties = [
+    {"party": "Progressive", "seats": 95, "color": "#306998"},
+    {"party": "Conservative", "seats": 82, "color": "#FFD43B"},
+    {"party": "Green", "seats": 45, "color": "#2CA02C"},
+    {"party": "Liberal", "seats": 38, "color": "#FF7F0E"},
+    {"party": "Social Dem.", "seats": 28, "color": "#9467BD"},
+    {"party": "Independent", "seats": 12, "color": "#8C564B"},
+]
+
+total_seats = sum(p["seats"] for p in parties)
+
+# Generate seat positions in semicircular arcs
+# Calculate number of rows based on total seats (more seats = more rows)
+n_rows = 5  # For 300 seats
+inner_radius = 3.0
+row_spacing = 1.0
+
+# Calculate seats per row (more seats in outer rows)
+row_weights = [(inner_radius + i * row_spacing) for i in range(n_rows)]
+total_weight = sum(row_weights)
+seats_per_row = [max(1, int(total_seats * w / total_weight)) for w in row_weights]
+
+# Adjust to match total exactly
+diff = total_seats - sum(seats_per_row)
+for i in range(abs(diff)):
+    if diff > 0:
+        seats_per_row[-(i % n_rows) - 1] += 1
+    else:
+        seats_per_row[i % n_rows] -= 1
+
+# Generate positions for each seat
+all_seats = []
+seat_idx = 0
+
+for row_idx, n_seats_in_row in enumerate(seats_per_row):
+    radius = inner_radius + row_idx * row_spacing
+    angles = np.linspace(np.pi, 0, n_seats_in_row)
+    for angle in angles:
+        x = radius * np.cos(angle)
+        y = radius * np.sin(angle)
+        all_seats.append({"x": x, "y": y, "seat_idx": seat_idx, "angle": angle})
+        seat_idx += 1
+
+# Sort seats by angle descending (pi to 0 = left to right)
+seats_sorted = sorted(all_seats, key=lambda s: -s["angle"])
+
+# Assign parties to seats in order
+seat_data = []
+current_idx = 0
+for party_info in parties:
+    party_name = party_info["party"]
+    party_color = party_info["color"]
+    n_seats = party_info["seats"]
+    for _ in range(n_seats):
+        if current_idx < len(seats_sorted):
+            seat = seats_sorted[current_idx]
+            seat_data.append(
+                {"x": seat["x"], "y": seat["y"], "party": party_name, "color": party_color, "seats_count": n_seats}
+            )
+            current_idx += 1
+
+df = pd.DataFrame(seat_data)
+
+# Create color scale from party data
+party_colors = {p["party"]: p["color"] for p in parties}
+color_domain = list(party_colors.keys())
+color_range = list(party_colors.values())
+
+# Create legend labels with seat counts
+legend_labels = {p["party"]: f"{p['party']} ({p['seats']})" for p in parties}
+df["party_label"] = df["party"].map(legend_labels)
+
+# Create the parliament chart
+chart = (
+    alt.Chart(df)
+    .mark_circle(size=250, opacity=0.9)
+    .encode(
+        x=alt.X("x:Q", axis=None),
+        y=alt.Y("y:Q", axis=None, scale=alt.Scale(domain=[-0.5, inner_radius + n_rows * row_spacing])),
+        color=alt.Color(
+            "party_label:N",
+            scale=alt.Scale(domain=[legend_labels[p["party"]] for p in parties], range=color_range),
+            legend=alt.Legend(
+                title="Parties",
+                titleFontSize=20,
+                labelFontSize=16,
+                labelLimit=200,
+                symbolSize=300,
+                orient="bottom",
+                direction="horizontal",
+                columns=3,
+                titleOrient="top",
+                titleAnchor="middle",
+            ),
+        ),
+        tooltip=["party:N", "seats_count:Q"],
+    )
+    .properties(
+        width=1500, height=800, title=alt.Title("parliament-basic · altair · pyplots.ai", fontSize=28, anchor="middle")
+    )
+    .configure_view(strokeWidth=0)
+    .configure_legend(padding=20, offset=0)
+)
+
+# Save as PNG and HTML
+chart.save("plot.png", scale_factor=3.0)
+chart.save("plot.html")

plots/parliament-basic/metadata/altair.yaml

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+library: altair
+specification_id: parliament-basic
+created: '2025-12-30T00:03:33Z'
+updated: '2025-12-30T00:20:19Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20585401343
+issue: 0
+python_version: 3.13.11
+library_version: 6.0.0
+preview_url: https://storage.googleapis.com/pyplots-images/plots/parliament-basic/altair/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/parliament-basic/altair/plot_thumb.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/parliament-basic/altair/plot.html
+quality_score: 90
+review:
+  strengths:
+  - Excellent semicircular parliament visualization with proper arc arrangement
+  - Seats correctly distributed with more seats in outer rows (realistic parliament
+    layout)
+  - Well-configured legend with party names and seat counts
+  - Good use of Altair declarative grammar with custom color scales
+  - Proper title format and good color accessibility
+  weaknesses:
+  - Large gap between the parliament chart and the legend creates visual disconnection
+  - Uses political party theme which content policy suggests avoiding for neutral
+    topics