feat(pygal): implement phase-diagram

[github-actions]

Implementation: phase-diagram - pygal

Implements the pygal version of phase-diagram.

File: plots/phase-diagram/implementations/pygal.py

Parent Issue: #3004

---

🤖 impl-generate workflow

[claude]

AI Review - Attempt 1/3

Image Description

The plot displays a phase diagram showing two oscillator trajectories on a white background. The title "phase-diagram · pygal · pyplots.ai" appears at the top in dark gray text. The X-axis is labeled "Position x" (range -1.2 to 2) and the Y-axis is labeled "Velocity dx/dt" (range -3 to 3). There are two main trajectories: (1) A blue spiral trajectory ("Damped Oscillator γ=0.3") that starts from the outer region and spirals inward toward the origin, demonstrating damped oscillation converging to equilibrium; (2) A yellow elliptical trajectory ("Undamped Oscillator (Limit Cycle)") that forms a closed loop, representing periodic motion without energy loss. A small red dot at the origin marks the "Equilibrium (Fixed Point)". The legend is displayed at the bottom with colored squares indicating each series. Grid lines are shown in light gray.

Quality Score: 91/100

Criteria Checklist

Visual Quality (35 pts)

• VQ-01: Text Legibility (9/10) - Title and axis labels are clearly readable with large font sizes appropriate for the 4800x2700 canvas. Tick labels are well-sized.
• VQ-02: No Overlap (8/8) - No overlapping text elements. Legend at bottom is cleanly separated.
• VQ-03: Element Visibility (7/8) - Line trajectories are smooth and visible with good stroke width. The equilibrium point marker at origin is small but visible.
• VQ-04: Color Accessibility (5/5) - Blue and yellow color combination is colorblind-safe, good contrast on white background.
• VQ-05: Layout Balance (4/5) - Good canvas utilization with plot filling most of the space. Slight asymmetry in axis ranges.
• VQ-06: Axis Labels (2/2) - Descriptive labels: "Position x" and "Velocity dx/dt".
• VQ-07: Grid & Legend (2/2) - Grid is subtle and non-intrusive. Legend is well-placed at bottom.

Spec Compliance (25 pts)

• SC-01: Plot Type (8/8) - Correct XY plot showing phase space trajectories.
• SC-02: Data Mapping (5/5) - Position (x) correctly on X-axis, derivative (dx/dt) on Y-axis.
• SC-03: Required Features (5/5) - Shows trajectory, fixed point at origin, demonstrates both damped spiral and limit cycle behaviors.
• SC-04: Data Range (3/3) - Axes show all trajectory data without clipping.
• SC-05: Legend Accuracy (2/2) - Legend entries accurately describe each series with parameters.
• SC-06: Title Format (2/2) - Follows correct format: "phase-diagram · pygal · pyplots.ai".

Data Quality (18 pts)

• DQ-01: Feature Coverage (7/8) - Shows damped spiral converging to equilibrium AND closed limit cycle. Could benefit from showing trajectories with different initial conditions.
• DQ-02: Realistic Context (7/7) - Damped harmonic oscillator is a classic physics example, realistic parameters (ω=2, γ=0.3).
• DQ-03: Appropriate Scale (4/5) - Values are physically sensible for a mechanical oscillator system.

Code Quality (9/10)

• CQ-01: KISS Structure (3/3) - Simple linear structure: imports → data → plot → save.
• CQ-02: Reproducibility (3/3) - Uses np.random.seed(42), though data is deterministic anyway.
• CQ-03: Clean Imports (2/2) - Only necessary imports: numpy, pygal, Style.
• CQ-04: No Deprecated API (1/1) - Uses current pygal API.
• CQ-05: Output Correct (0/1) - Saves both plot.html and plot.png correctly.

Library Features (4/5)

• LF-01: Uses distinctive library features (4/5) - Good use of pygal's XY chart, custom Style configuration, show_dots option for line-only rendering, legend_at_bottom.

Strengths

• Excellent demonstration of phase space concepts with both damped and undamped trajectories
• Clean visual distinction between the spiral (damped) and closed loop (limit cycle) behaviors
• Well-chosen colorblind-safe color scheme (blue and yellow)
• Proper mathematical implementation of damped harmonic oscillator using Euler integration
• Good use of pygal custom styling for large canvas rendering

Weaknesses

• The equilibrium point marker at origin is quite small and could be more prominent
• The undamped oscillator trajectory shows some numerical drift (not a perfect ellipse due to Euler method)

Verdict: APPROVED