feat(pygal): implement network-force-directed (#5438)

## Implementation: `network-force-directed` - python/pygal

Implements the **python/pygal** version of `network-force-directed`.

**File:** `plots/network-force-directed/implementations/python/pygal.py`

**Parent Issue:** #990

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/anyplot/actions/runs/24952933179)*

---------

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

Author: github-actions[bot]

plots/network-force-directed/implementations/python/pygal.py

@@ -1,23 +1,42 @@
-""" pyplots.ai
+""" anyplot.ai
 network-force-directed: Force-Directed Graph
-Library: pygal 3.1.0 | Python 3.13.11
-Quality: 92/100 | Created: 2025-12-17
+Library: pygal 3.1.0 | Python 3.14.4
+Quality: 83/100 | Created: 2026-04-26
 """
 
-import numpy as np
-import pygal
-from pygal.style import Style
+import sys
+from pathlib import Path
 
 
-# Set seed for reproducibility
+# Remove script directory from path to avoid name collision with pygal package
+_script_dir = str(Path(__file__).parent)
+sys.path = [p for p in sys.path if p != _script_dir]
+
+import os  # noqa: E402
+
+import numpy as np  # noqa: E402
+import pygal  # noqa: E402
+from pygal.style import Style  # noqa: E402
+
+
+# Theme-adaptive tokens
+THEME = os.getenv("ANYPLOT_THEME", "light")
+PAGE_BG = "#FAF8F1" if THEME == "light" else "#1A1A17"
+INK = "#1A1A17" if THEME == "light" else "#F0EFE8"
+INK_SOFT = "#4A4A44" if THEME == "light" else "#B8B7B0"
+INK_MUTED = "#6B6A63" if THEME == "light" else "#A8A79F"
+EDGE_COLOR = "#9A988F" if THEME == "light" else "#5A5852"
+
+OKABE_ITO = ("#009E73", "#D55E00", "#0072B2", "#CC79A7", "#E69F00", "#56B4E9", "#F0E442")
+
+# Reproducibility
 np.random.seed(42)
 
-# Data: A social network with 50 nodes in 3 communities
+# Data: A corporate social network with 50 nodes in 3 departments
 # Demonstrates force-directed layout with clear community structure
 nodes = []
 edges = []
 
-# Create 3 communities
 community_sizes = [18, 17, 15]  # Total: 50 nodes
 community_names = ["Engineering", "Marketing", "Sales"]
 node_id = 0
@@ -50,18 +69,17 @@
 bridge_edges = [(0, 18), (5, 20), (10, 25), (18, 35), (22, 40), (30, 45), (8, 38), (15, 48)]
 edges.extend(bridge_edges)
 
-# Force-directed layout algorithm (Fruchterman-Reingold)
+# Force-directed layout (Fruchterman-Reingold)
 n = len(nodes)
 positions = np.random.rand(n, 2) * 2 - 1  # Initial random positions
 
-# Optimal distance parameter
-k = 0.5
-iterations = 200
+k = 0.95  # Optimal distance — larger to reduce dense-cluster node overlap
+iterations = 320
 
 for iteration in range(iterations):
     displacement = np.zeros((n, 2))
 
-    # Repulsive forces between all node pairs (nodes push apart)
+    # Repulsive forces between all node pairs
     for i in range(n):
         for j in range(i + 1, n):
             diff = positions[i] - positions[j]
@@ -70,126 +88,108 @@
             displacement[i] += repulsive_force
             displacement[j] -= repulsive_force
 
-    # Attractive forces along edges (connected nodes pull together)
+    # Attractive forces along edges
     for src, tgt in edges:
         diff = positions[src] - positions[tgt]
         dist = max(np.linalg.norm(diff), 0.01)
         attractive_force = (dist * dist / k) * (diff / dist)
         displacement[src] -= attractive_force
         displacement[tgt] += attractive_force
 
-    # Apply displacement with cooling (decreasing temperature)
+    # Apply displacement with cooling
     temperature = 1 - iteration / iterations
     for i in range(n):
         disp_norm = np.linalg.norm(displacement[i])
         if disp_norm > 0:
-            # Limit movement by temperature
             positions[i] += (displacement[i] / disp_norm) * min(disp_norm, 0.15 * temperature)
 
-# Normalize positions to [1, 11] range for pygal (with padding)
+# Normalize positions to a padded plotting range
 pos_min = positions.min(axis=0)
 pos_max = positions.max(axis=0)
 positions = (positions - pos_min) / (pos_max - pos_min + 1e-6) * 10 + 1
 pos = {node["id"]: positions[i] for i, node in enumerate(nodes)}
 
-# Calculate node degrees (number of connections)
+# Node degrees (for tooltip context)
 degrees = {node["id"]: 0 for node in nodes}
 for src, tgt in edges:
     degrees[src] += 1
     degrees[tgt] += 1
 
-# Community colors
-community_colors = ["#306998", "#FFD43B", "#FF6B6B"]
+# Style — first data series is the edge "Connections" (muted), then communities use Okabe-Ito 1..3
+community_colors = OKABE_ITO[: len(community_names)]
+series_colors = (EDGE_COLOR,) + community_colors
 
-# Custom style for the chart
 custom_style = Style(
-    background="white",
-    plot_background="white",
-    foreground="#333333",
-    foreground_strong="#333333",
-    foreground_subtle="#666666",
-    colors=("#AAAAAA",) + tuple(community_colors),
+    background=PAGE_BG,
+    plot_background=PAGE_BG,
+    foreground=INK,
+    foreground_strong=INK,
+    foreground_subtle=INK_MUTED,
+    colors=series_colors,
     title_font_size=72,
     label_font_size=40,
     major_label_font_size=36,
-    legend_font_size=40,
+    legend_font_size=44,
     value_font_size=32,
     stroke_width=2,
-    opacity=0.85,
+    opacity=0.9,
     opacity_hover=1.0,
+    tooltip_font_size=28,
+    font_family="DejaVu Sans, Helvetica, Arial, sans-serif",
 )
 
-# Create XY chart
 chart = pygal.XY(
     width=4800,
     height=2700,
     style=custom_style,
-    title="network-force-directed · pygal · pyplots.ai",
+    title="network-force-directed · pygal · anyplot.ai",
     show_legend=True,
-    x_title="",
-    y_title="",
     show_x_guides=False,
     show_y_guides=False,
     show_x_labels=False,
     show_y_labels=False,
     stroke=True,
-    dots_size=25,
+    dots_size=28,
     stroke_style={"width": 1.5, "linecap": "round"},
     legend_at_bottom=True,
     legend_at_bottom_columns=4,
+    legend_box_size=36,
+    margin=80,
     range=(0, 12),
     xrange=(0, 12),
 )
 
-# Add edges as a single series with lines connecting pairs
-# Each edge is represented as two points connected, with None to break between edges
+# Edges as a single XY series with None breaks between segments
 edge_points = []
 for src, tgt in edges:
     x1, y1 = pos[src]
     x2, y2 = pos[tgt]
     edge_points.append((x1, y1))
     edge_points.append((x2, y2))
-    edge_points.append(None)  # Break the line for next edge
+    edge_points.append(None)
 
 chart.add("Connections", edge_points, stroke=True, show_dots=False, fill=False)
 
-# Add nodes grouped by community
+# Nodes grouped by community — radius scales with node degree (visual encoding)
+# pygal supports per-point SVG attribute overrides via the "node" dict
+max_degree = max(degrees.values())
+min_radius, max_radius = 18, 52
 for comm_idx, comm_name in enumerate(community_names):
     comm_nodes = [node for node in nodes if node["community"] == comm_idx]
-    # Create points with labels for tooltips showing degree
     node_points = []
     for node in comm_nodes:
         x, y = pos[node["id"]]
         degree = degrees[node["id"]]
+        radius = min_radius + (max_radius - min_radius) * (degree / max_degree)
         label = f"Node {node['id']} | {degree} connections"
         if degree >= 7:
             label += " (Hub)"
-        node_points.append({"value": (x, y), "label": label})
+        node_points.append({"value": (x, y), "label": label, "node": {"r": round(radius, 1)}})
     chart.add(comm_name, node_points, stroke=False)
 
-# Save outputs
-chart.render_to_file("plot.svg")
-chart.render_to_png("plot.png")
-
-# Also save HTML for interactive version
-with open("plot.html", "w") as f:
-    f.write(
-        """<!DOCTYPE html>
-<html>
-<head>
-    <title>network-force-directed · pygal · pyplots.ai</title>
-    <style>
-        body { margin: 0; padding: 20px; background: #f5f5f5; }
-        .container { max-width: 100%; margin: 0 auto; }
-        object { width: 100%; height: auto; }
-    </style>
-</head>
-<body>
-    <div class="container">
-        <object type="image/svg+xml" data="plot.svg">
-            Force-directed network graph not supported
-        </object>
-    </div>
-</body>
-</html>"""
-    )
+# Save outputs (theme-aware filenames)
+chart.render_to_file(f"plot-{THEME}.svg")
+chart.render_to_png(f"plot-{THEME}.png")
+
+with open(f"plot-{THEME}.html", "wb") as f:
+    f.write(chart.render())