feat(pygal): implement streamline-basic (#2913)

## Implementation: `streamline-basic` - pygal

Implements the **pygal** version of `streamline-basic`.

**File:** `plots/streamline-basic/implementations/pygal.py`

**Parent Issue:** #2861

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20608714729)*

---------

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/streamline-basic/implementations/pygal.py

@@ -0,0 +1,126 @@
+""" pyplots.ai
+streamline-basic: Basic Streamline Plot
+Library: pygal 3.1.0 | Python 3.13.11
+Quality: 77/100 | Created: 2025-12-31
+"""
+
+import numpy as np
+import pygal
+from pygal.style import Style
+
+
+# Data - Vortex flow field (circular streamlines)
+# u = -y, v = x creates counterclockwise circular flow
+np.random.seed(42)
+
+# Trace streamlines inline - KISS structure (no helper functions)
+# Starting points distributed at different radii to show flow structure
+streamlines = []
+
+# Use 4 radii to show different orbital distances
+radii = [0.8, 1.3, 1.8, 2.3]
+points_per_radius = 5
+
+for radius in radii:
+    for angle_idx in range(points_per_radius):
+        angle = 2 * np.pi * angle_idx / points_per_radius
+        x0 = radius * np.cos(angle)
+        y0 = radius * np.sin(angle)
+
+        # Trace streamline from this starting point
+        points = [(x0, y0)]
+        x, y = x0, y0
+        dt = 0.02  # Smaller step for smoother curves
+        max_steps = 400  # More steps for longer, smoother curves
+        bounds = 3.0
+
+        for _ in range(max_steps):
+            # Velocity field: vortex with radial decay
+            r = np.sqrt(x**2 + y**2)
+            factor = 1.0 / (1.0 + 0.1 * r)
+            u = -y * factor
+            v = x * factor
+            speed = np.sqrt(u**2 + v**2)
+
+            if speed < 0.001:  # Stagnation point
+                break
+
+            # Normalize and step
+            x_new = x + dt * u / speed
+            y_new = y + dt * v / speed
+
+            # Check bounds
+            if abs(x_new) > bounds or abs(y_new) > bounds:
+                break
+
+            x, y = x_new, y_new
+            points.append((x, y))
+
+        if len(points) > 10:  # Only keep meaningful streamlines
+            streamlines.append((points, radius))
+
+# Group streamlines by radial distance for coloring
+# 4 distinct high-contrast colors for different radii
+# Labels describe orbit radius, not "speed" - for vortex flow, all orbits have same
+# angular velocity so inner orbits have lower linear speed
+bin_colors = ["#d62728", "#2ca02c", "#1f77b4", "#9467bd"]
+bin_labels = ["Inner Orbit (r=0.8)", "Mid-Inner Orbit (r=1.3)", "Mid-Outer Orbit (r=1.8)", "Outer Orbit (r=2.3)"]
+
+binned_streamlines = {i: [] for i in range(4)}
+for points, radius in streamlines:
+    bin_idx = radii.index(radius)
+    binned_streamlines[bin_idx].append(points)
+
+# Custom style with larger fonts for readability
+custom_style = Style(
+    background="white",
+    plot_background="white",
+    foreground="#333333",
+    foreground_strong="#333333",
+    foreground_subtle="#555555",
+    colors=tuple(bin_colors),
+    title_font_size=72,
+    label_font_size=48,
+    major_label_font_size=40,
+    legend_font_size=48,
+    value_font_size=32,
+    guide_stroke_color="#cccccc",
+)
+
+# Create chart
+chart = pygal.XY(
+    style=custom_style,
+    width=4800,
+    height=2700,
+    stroke=True,
+    stroke_style={"width": 10},
+    show_dots=False,
+    show_legend=True,
+    legend_at_bottom=True,
+    legend_at_bottom_columns=4,
+    title="streamline-basic · pygal · pyplots.ai",
+    x_title="X Position",
+    y_title="Y Position",
+    show_x_guides=True,
+    show_y_guides=True,
+    range=(-3.5, 3.5),
+    xrange=(-3.5, 3.5),
+    dots_size=8,  # Larger dots for legend visibility
+)
+
+# Add each bin as a series
+for bin_idx in range(4):
+    series_data = []
+    for sl in binned_streamlines[bin_idx]:
+        # Add streamline points
+        for point in sl:
+            series_data.append(point)
+        # Add None to separate streamlines
+        series_data.append(None)
+
+    if series_data:
+        chart.add(bin_labels[bin_idx], series_data)
+
+# Save outputs
+chart.render_to_png("plot.png")
+chart.render_to_file("plot.html")

plots/streamline-basic/metadata/pygal.yaml

@@ -0,0 +1,30 @@
+library: pygal
+specification_id: streamline-basic
+created: '2025-12-31T00:16:07Z'
+updated: '2025-12-31T00:35:04Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20608714729
+issue: 2861
+python_version: 3.13.11
+library_version: 3.1.0
+preview_url: https://storage.googleapis.com/pyplots-images/plots/streamline-basic/pygal/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/streamline-basic/pygal/plot_thumb.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/streamline-basic/pygal/plot.html
+quality_score: 77
+review:
+  strengths:
+  - Creative workaround for pygal lack of native streamline support using XY connected
+    lines
+  - Good visual representation of vortex flow with distinct colored orbital groups
+  - Clean code structure following KISS principles
+  - Appropriate use of custom Style for font sizing and colors
+  - Good canvas utilization with balanced layout
+  - Correct title format
+  weaknesses:
+  - Legend labels in rendered image appear truncated compared to code-defined labels
+  - The blue color tones (Near-Inner and Middle) are somewhat similar and could be
+    more distinct
+  - Streamlines only show closed circular orbits - more varied flow features would
+    better demonstrate streamline capabilities
+  - Random seed is set but not actually needed since the vortex field is mathematically
+    deterministic