feat(plotnine): implement venn-basic (#2477)

## Implementation: `venn-basic` - plotnine

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

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

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20584330726)*

---------

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/venn-basic/implementations/plotnine.py

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+""" pyplots.ai
+venn-basic: Venn Diagram
+Library: plotnine 0.15.2 | Python 3.13.11
+Quality: 91/100 | Created: 2025-12-29
+"""
+
+import math
+
+import numpy as np
+import pandas as pd
+from plotnine import (
+    aes,
+    element_blank,
+    element_rect,
+    element_text,
+    geom_polygon,
+    geom_text,
+    ggplot,
+    labs,
+    scale_fill_manual,
+    scale_x_continuous,
+    scale_y_continuous,
+    theme,
+)
+
+
+# Data - Three overlapping sets representing skills in a tech team
+# Set A: Python developers (100 people)
+# Set B: Data Scientists (80 people)
+# Set C: ML Engineers (60 people)
+# Overlaps: A∩B=30, A∩C=20, B∩C=25, A∩B∩C=10
+
+set_labels = ["Python\nDevelopers", "Data\nScientists", "ML\nEngineers"]
+# Region counts (exclusive to each region)
+# A only: 100 - 30 - 20 + 10 = 60
+# B only: 80 - 30 - 25 + 10 = 35
+# C only: 60 - 20 - 25 + 10 = 25
+# A∩B only: 30 - 10 = 20
+# A∩C only: 20 - 10 = 10
+# B∩C only: 25 - 10 = 15
+# A∩B∩C: 10
+
+region_counts = {"A_only": 60, "B_only": 35, "C_only": 25, "AB_only": 20, "AC_only": 10, "BC_only": 15, "ABC": 10}
+
+
+# Circle positions for 3-set Venn diagram
+# Circles arranged in a triangle formation with significant overlap
+radius = 5
+offset = radius * 0.6  # Controls overlap amount
+n_points = 100
+
+# Circle centers
+centers = {
+    "A": (-offset, offset * 0.5),  # Top-left
+    "B": (offset, offset * 0.5),  # Top-right
+    "C": (0, -offset * 0.9),  # Bottom
+}
+
+# Colors with transparency for overlap visibility
+colors = {
+    "A": "#306998",  # Python Blue
+    "B": "#FFD43B",  # Python Yellow
+    "C": "#4B8BBE",  # Lighter blue
+}
+
+# Create circle polygons
+circle_rows = []
+circle_id = 0
+for label, (cx, cy) in centers.items():
+    angles = np.linspace(0, 2 * math.pi, n_points + 1)
+    x_coords = cx + radius * np.cos(angles)
+    y_coords = cy + radius * np.sin(angles)
+    for i in range(len(x_coords)):
+        circle_rows.append({"x": x_coords[i], "y": y_coords[i], "circle_id": circle_id, "set": label})
+    circle_id += 1
+
+circle_df = pd.DataFrame(circle_rows)
+
+# Calculate positions for region labels
+# A only - left side of circle A
+label_A_only = {
+    "x": centers["A"][0] - radius * 0.45,
+    "y": centers["A"][1] + radius * 0.2,
+    "label": str(region_counts["A_only"]),
+}
+
+# B only - right side of circle B
+label_B_only = {
+    "x": centers["B"][0] + radius * 0.45,
+    "y": centers["B"][1] + radius * 0.2,
+    "label": str(region_counts["B_only"]),
+}
+
+# C only - bottom of circle C
+label_C_only = {"x": centers["C"][0], "y": centers["C"][1] - radius * 0.5, "label": str(region_counts["C_only"])}
+
+# AB intersection (top center)
+label_AB = {"x": 0, "y": centers["A"][1] + radius * 0.35, "label": str(region_counts["AB_only"])}
+
+# AC intersection (bottom-left)
+label_AC = {
+    "x": centers["A"][0] + radius * 0.35,
+    "y": (centers["A"][1] + centers["C"][1]) / 2 - radius * 0.1,
+    "label": str(region_counts["AC_only"]),
+}
+
+# BC intersection (bottom-right)
+label_BC = {
+    "x": centers["B"][0] - radius * 0.35,
+    "y": (centers["B"][1] + centers["C"][1]) / 2 - radius * 0.1,
+    "label": str(region_counts["BC_only"]),
+}
+
+# ABC intersection (center)
+centroid_x = (centers["A"][0] + centers["B"][0] + centers["C"][0]) / 3
+centroid_y = (centers["A"][1] + centers["B"][1] + centers["C"][1]) / 3
+label_ABC = {"x": centroid_x, "y": centroid_y, "label": str(region_counts["ABC"])}
+
+# Create label dataframes
+count_labels_df = pd.DataFrame([label_A_only, label_B_only, label_C_only, label_AB, label_AC, label_BC, label_ABC])
+
+# Set name labels - positioned outside circles
+set_name_labels = [
+    {"x": centers["A"][0] - radius * 0.8, "y": centers["A"][1] + radius * 0.9, "label": set_labels[0]},
+    {"x": centers["B"][0] + radius * 0.8, "y": centers["B"][1] + radius * 0.9, "label": set_labels[1]},
+    {"x": centers["C"][0], "y": centers["C"][1] - radius * 1.1, "label": set_labels[2]},
+]
+set_name_df = pd.DataFrame(set_name_labels)
+
+# Plot
+plot = (
+    ggplot()
+    # Draw circles with transparency for overlap visibility
+    + geom_polygon(
+        aes(x="x", y="y", fill="set", group="circle_id"), data=circle_df, alpha=0.45, color="#333333", size=1.5
+    )
+    # Region count labels (larger, bold)
+    + geom_text(aes(x="x", y="y", label="label"), data=count_labels_df, size=18, fontweight="bold", color="#000000")
+    # Set name labels (outside circles)
+    + geom_text(aes(x="x", y="y", label="label"), data=set_name_df, size=14, fontweight="bold", color="#333333")
+    # Colors (legend hidden via theme)
+    + scale_fill_manual(values=colors)
+    # Axis scaling for proper aspect ratio
+    + scale_x_continuous(limits=(-12, 12))
+    + scale_y_continuous(limits=(-12, 10))
+    # Title
+    + labs(title="venn-basic · plotnine · pyplots.ai")
+    # Clean theme
+    + theme(
+        figure_size=(12, 12),
+        plot_title=element_text(size=24, ha="center", color="#333333"),
+        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_rect(fill="#FFFFFF"),
+        plot_background=element_rect(fill="#FFFFFF"),
+        legend_position="none",  # Hide legend - labels are on the plot
+    )
+)
+
+# Save
+plot.save("plot.png", dpi=300)

plots/venn-basic/metadata/plotnine.yaml

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+library: plotnine
+specification_id: venn-basic
+created: '2025-12-29T22:49:39Z'
+updated: '2025-12-29T22:58:31Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20584330726
+issue: 0
+python_version: 3.13.11
+library_version: 0.15.2
+preview_url: https://storage.googleapis.com/pyplots-images/plots/venn-basic/plotnine/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/venn-basic/plotnine/plot_thumb.png
+preview_html: null
+quality_score: 91
+review:
+  strengths:
+  - Excellent implementation of a Venn diagram using plotnine grammar of graphics
+    approach
+  - Clean, readable layout with well-positioned count labels in each region
+  - Good use of transparency (alpha=0.45) to show overlap regions clearly
+  - Realistic tech team scenario with mathematically consistent intersection counts
+  - Proper calculation of exclusive region counts from set totals and intersections
+  - Clean theme with hidden axes and grid appropriate for a Venn diagram
+  weaknesses:
+  - 'Two of the three colors are shades of blue (#306998 and #4B8BBE), which could
+    be confusing for colorblind users'
+  - Figure size is 12x12 (square) instead of 16x9 as recommended in library guidelines