feat(plotnine): implement line-parametric (#5071)

## Implementation: `line-parametric` - plotnine

Implements the **plotnine** version of `line-parametric`.

**File:** `plots/line-parametric/implementations/plotnine.py`

**Parent Issue:** #4424

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/23339035321)*

---------

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/line-parametric/implementations/plotnine.py

@@ -0,0 +1,127 @@
+""" pyplots.ai
+line-parametric: Parametric Curve Plot
+Library: plotnine 0.15.3 | Python 3.14.3
+Quality: 92/100 | Created: 2026-03-20
+"""
+
+import numpy as np
+import pandas as pd
+from plotnine import (
+    aes,
+    coord_equal,
+    element_blank,
+    element_line,
+    element_rect,
+    element_text,
+    facet_wrap,
+    geom_path,
+    geom_point,
+    ggplot,
+    guide_colorbar,
+    guide_legend,
+    guides,
+    labs,
+    scale_color_gradientn,
+    scale_fill_manual,
+    scale_shape_manual,
+    theme,
+    theme_void,
+)
+
+
+# Data — normalize t to [0, 1] per curve so both panels use the full color gradient
+n_points = 1000
+t_lissajous = np.linspace(0, 2 * np.pi, n_points)
+x_lissajous = np.sin(3 * t_lissajous)
+y_lissajous = np.sin(2 * t_lissajous)
+t_norm_liss = np.linspace(0, 1, n_points)
+
+t_spiral = np.linspace(0, 4 * np.pi, n_points)
+x_spiral = t_spiral * np.cos(t_spiral) / (4 * np.pi)
+y_spiral = t_spiral * np.sin(t_spiral) / (4 * np.pi)
+t_norm_spiral = np.linspace(0, 1, n_points)
+
+df = pd.concat(
+    [
+        pd.DataFrame(
+            {"x": x_lissajous, "y": y_lissajous, "t_norm": t_norm_liss, "curve": "Lissajous · x = sin(3t), y = sin(2t)"}
+        ),
+        pd.DataFrame(
+            {"x": x_spiral, "y": y_spiral, "t_norm": t_norm_spiral, "curve": "Spiral · x = t·cos(t), y = t·sin(t)"}
+        ),
+    ],
+    ignore_index=True,
+)
+
+# Start and end markers
+start_pts = pd.DataFrame(
+    {
+        "x": [x_lissajous[0], x_spiral[0]],
+        "y": [y_lissajous[0], y_spiral[0]],
+        "t_norm": [0.0, 0.0],
+        "curve": ["Lissajous · x = sin(3t), y = sin(2t)", "Spiral · x = t·cos(t), y = t·sin(t)"],
+        "endpoint": ["Start (t = 0)", "Start (t = 0)"],
+    }
+)
+
+end_pts = pd.DataFrame(
+    {
+        "x": [x_lissajous[-1], x_spiral[-1]],
+        "y": [y_lissajous[-1], y_spiral[-1]],
+        "t_norm": [1.0, 1.0],
+        "curve": ["Lissajous · x = sin(3t), y = sin(2t)", "Spiral · x = t·cos(t), y = t·sin(t)"],
+        "endpoint": ["End (t = tmax)", "End (t = tmax)"],
+    }
+)
+
+markers = pd.concat([start_pts, end_pts], ignore_index=True)
+
+# Gradient palette — deep navy through teal, gold, to vivid rose
+gradient_colors = ["#0d1b2a", "#1b4965", "#5fa8d3", "#bee9e8", "#ffd166", "#ef476f"]
+
+# Plot
+plot = (
+    ggplot(df, aes(x="x", y="y", color="t_norm"))
+    + geom_path(aes(group="curve"), size=2.5, alpha=0.94)
+    + geom_point(
+        aes(shape="endpoint", fill="endpoint"), data=markers, color="#1a1a2e", size=8, stroke=1.2, show_legend=True
+    )
+    + scale_shape_manual(name="Direction", values={"Start (t = 0)": "o", "End (t = tmax)": "D"})
+    + scale_fill_manual(name="Direction", values={"Start (t = 0)": "#306998", "End (t = tmax)": "#ef476f"})
+    + facet_wrap("curve", scales="free")
+    + scale_color_gradientn(
+        name="Parameter t",
+        colors=gradient_colors,
+        guide=guide_colorbar(nbin=200),
+        labels=["0", "¼", "½", "¾", "tmax"],
+        breaks=[0.0, 0.25, 0.5, 0.75, 1.0],
+    )
+    + coord_equal()
+    + labs(title="line-parametric · plotnine · pyplots.ai", x="Horizontal Position  x(t)", y="Vertical Position  y(t)")
+    + guides(shape=guide_legend(order=2), fill=guide_legend(order=2))
+    + theme_void()
+    + theme(
+        figure_size=(16, 9),
+        plot_title=element_text(size=26, weight="bold", color="#0d1b2a", margin={"b": 14}),
+        axis_title_x=element_text(size=20, color="#333333", margin={"t": 10}),
+        axis_title_y=element_text(size=20, color="#333333", margin={"r": 10}),
+        axis_text=element_text(size=16, color="#555555"),
+        axis_ticks=element_blank(),
+        legend_title=element_text(size=17, weight="bold", color="#0d1b2a"),
+        legend_text=element_text(size=16, color="#333333"),
+        legend_key=element_rect(fill="white", color="white"),
+        legend_background=element_rect(fill="#fafafa", color="#e0e0e0", size=0.5),
+        legend_box_margin=6,
+        strip_text=element_text(size=17, weight="bold", color="#0d1b2a", margin={"b": 8}),
+        strip_background=element_rect(fill="#f0f4f8", color="none"),
+        panel_spacing_x=0.15,
+        panel_grid_major=element_line(color="#e8e8e8", size=0.3, linetype="dashed"),
+        panel_grid_minor=element_blank(),
+        panel_background=element_rect(fill="white", color="none"),
+        plot_background=element_rect(fill="white", color="none"),
+        plot_margin=0.01,
+    )
+)
+
+# Save
+plot.save("plot.png", dpi=300, verbose=False)

plots/line-parametric/metadata/plotnine.yaml

@@ -0,0 +1,225 @@
+library: plotnine
+specification_id: line-parametric
+created: '2026-03-20T10:36:50Z'
+updated: '2026-03-20T10:58:58Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 23339035321
+issue: 4424
+python_version: 3.14.3
+library_version: 0.15.3
+preview_url: https://storage.googleapis.com/pyplots-images/plots/line-parametric/plotnine/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/line-parametric/plotnine/plot_thumb.png
+preview_html: null
+quality_score: 92
+review:
+  strengths:
+  - 'Excellent spec compliance: all required features (equal aspect ratio, color gradient,
+    two curves, start/end markers) present and correct'
+  - Custom gradient palette creates visually appealing and informative progression
+    from navy to rose
+  - Idiomatic plotnine usage with proper grammar of graphics layering
+  - Clean deterministic code with no unnecessary complexity
+  weaknesses:
+  - 'Minor canvas utilization: spiral panel has some wasted whitespace due to coord_equal
+    with spiral smaller extent versus endpoint position'
+  image_description: The plot displays two parametric curves in a side-by-side faceted
+    layout on a white background. The left panel shows a Lissajous figure (x=sin(3t),
+    y=sin(2t)) — a self-intersecting figure-eight-like pattern spanning [-1, 1] on
+    both axes. The right panel shows a spiral curve (x=t·cos(t), y=t·sin(t)) that
+    winds outward from the origin. Both curves use a continuous color gradient from
+    deep navy (t=0) through teal, light cyan, gold/yellow, to vivid pink/rose (t=tmax)
+    to indicate the direction of parameter progression. Start points are marked with
+    dark blue filled circles, end points with pink diamonds. The title reads "line-parametric
+    · plotnine · pyplots.ai" in bold dark text. Axis labels are "Horizontal Position
+    x(t)" and "Vertical Position y(t)". A "Direction" legend on the right shows the
+    start/end marker shapes, and a "Parameter t" colorbar displays the full gradient
+    scale with labels 0, ¼, ½, ¾, tmax. Facet strip labels show the curve equations.
+    Subtle dashed grid lines appear on a clean white background.
+  criteria_checklist:
+    visual_quality:
+      score: 29
+      max: 30
+      items:
+      - id: VQ-01
+        name: Text Legibility
+        score: 8
+        max: 8
+        passed: true
+        comment: 'All font sizes explicitly set: title=26pt, axis_title=20pt, axis_text=16pt,
+          legend_title=17pt, strip_text=17pt'
+      - id: VQ-02
+        name: No Overlap
+        score: 6
+        max: 6
+        passed: true
+        comment: No overlapping text anywhere
+      - id: VQ-03
+        name: Element Visibility
+        score: 6
+        max: 6
+        passed: true
+        comment: Curves at size=2.5 with alpha=0.94, markers at size=8 with stroke
+          clearly visible
+      - id: VQ-04
+        name: Color Accessibility
+        score: 4
+        max: 4
+        passed: true
+        comment: Navy-teal-gold-rose gradient is colorblind-friendly
+      - id: VQ-05
+        name: Layout & Canvas
+        score: 3
+        max: 4
+        passed: true
+        comment: Good two-panel layout, minor whitespace in spiral panel due to coord_equal
+      - id: VQ-06
+        name: Axis Labels & Title
+        score: 2
+        max: 2
+        passed: true
+        comment: Descriptive labels with mathematical notation appropriate for domain
+    design_excellence:
+      score: 15
+      max: 20
+      items:
+      - id: DE-01
+        name: Aesthetic Sophistication
+        score: 6
+        max: 8
+        passed: true
+        comment: Custom 6-color gradient palette, thoughtful typography, intentional
+          color hierarchy
+      - id: DE-02
+        name: Visual Refinement
+        score: 5
+        max: 6
+        passed: true
+        comment: theme_void base with subtle dashed grid, custom legend styling, clean
+          backgrounds
+      - id: DE-03
+        name: Data Storytelling
+        score: 4
+        max: 6
+        passed: true
+        comment: Color gradient tells progression story, start/end markers guide viewer,
+          equations in facet labels
+    spec_compliance:
+      score: 15
+      max: 15
+      items:
+      - id: SC-01
+        name: Plot Type
+        score: 5
+        max: 5
+        passed: true
+        comment: Correct parametric curve plot with two example curves
+      - id: SC-02
+        name: Required Features
+        score: 4
+        max: 4
+        passed: true
+        comment: Equal aspect ratio, color gradient, two curves, start/end markers,
+          sufficient density
+      - id: SC-03
+        name: Data Mapping
+        score: 3
+        max: 3
+        passed: true
+        comment: x(t) and y(t) correctly computed and mapped
+      - id: SC-04
+        name: Title & Legend
+        score: 3
+        max: 3
+        passed: true
+        comment: Correct title format, clear legend labels
+    data_quality:
+      score: 14
+      max: 15
+      items:
+      - id: DQ-01
+        name: Feature Coverage
+        score: 6
+        max: 6
+        passed: true
+        comment: Self-intersecting closed curve and open expanding spiral demonstrate
+          key parametric behaviors
+      - id: DQ-02
+        name: Realistic Context
+        score: 4
+        max: 5
+        passed: true
+        comment: Well-known mathematical curves, neutral scientific context
+      - id: DQ-03
+        name: Appropriate Scale
+        score: 4
+        max: 4
+        passed: true
+        comment: Mathematically correct values, appropriate t ranges
+    code_quality:
+      score: 10
+      max: 10
+      items:
+      - id: CQ-01
+        name: KISS Structure
+        score: 3
+        max: 3
+        passed: true
+        comment: Clean Imports-Data-Plot-Save structure, no functions/classes
+      - id: CQ-02
+        name: Reproducibility
+        score: 2
+        max: 2
+        passed: true
+        comment: Fully deterministic mathematical functions
+      - id: CQ-03
+        name: Clean Imports
+        score: 2
+        max: 2
+        passed: true
+        comment: All imports used
+      - id: CQ-04
+        name: Code Elegance
+        score: 2
+        max: 2
+        passed: true
+        comment: Clean, well-organized, appropriate complexity
+      - id: CQ-05
+        name: Output & API
+        score: 1
+        max: 1
+        passed: true
+        comment: Saves as plot.png with dpi=300, current API
+    library_mastery:
+      score: 9
+      max: 10
+      items:
+      - id: LM-01
+        name: Idiomatic Usage
+        score: 5
+        max: 5
+        passed: true
+        comment: 'Expert grammar of graphics usage: aes, geom_path, facet_wrap, scale_color_gradientn,
+          coord_equal'
+      - id: LM-02
+        name: Distinctive Features
+        score: 4
+        max: 5
+        passed: true
+        comment: ggplot-specific layering, facet_wrap, guide_colorbar, scale_shape_manual
+          + scale_fill_manual
+  verdict: APPROVED
+impl_tags:
+  dependencies: []
+  techniques:
+  - faceting
+  - layer-composition
+  - colorbar
+  patterns:
+  - data-generation
+  dataprep:
+  - normalization
+  styling:
+  - minimal-chrome
+  - custom-colormap
+  - alpha-blending
+  - grid-styling