diff --git a/plots/line-reaction-coordinate/implementations/pygal.py b/plots/line-reaction-coordinate/implementations/pygal.py
new file mode 100644
index 0000000000..749efea9a5
--- /dev/null
+++ b/plots/line-reaction-coordinate/implementations/pygal.py
@@ -0,0 +1,166 @@
+""" pyplots.ai
+line-reaction-coordinate: Reaction Coordinate Energy Diagram
+Library: pygal 3.1.0 | Python 3.14.3
+Quality: 85/100 | Created: 2026-03-21
+"""
+
+import numpy as np
+import pygal
+from pygal.style import Style
+
+
+# Data - Single-step exothermic reaction
+reactant_energy = 50.0  # kJ/mol
+transition_energy = 120.0  # kJ/mol
+product_energy = 20.0  # kJ/mol
+activation_energy = transition_energy - reactant_energy  # Ea = 70 kJ/mol
+enthalpy_change = product_energy - reactant_energy  # ΔH = -30 kJ/mol
+
+# Generate smooth energy profile
+n_points = 300
+reaction_coord = np.linspace(0, 10, n_points)
+
+# Gaussian barrier centered at transition state
+sigma = 1.2
+peak_pos = 5.0
+base_curve = reactant_energy + (transition_energy - reactant_energy) * np.exp(
+    -0.5 * ((reaction_coord - peak_pos) / sigma) ** 2
+)
+
+# Vectorized Hermite smoothstep transition to product energy
+t_raw = np.clip((reaction_coord - 6.5) / 2.0, 0.0, 1.0)
+smooth_t = t_raw * t_raw * (3 - 2 * t_raw)
+base_curve = base_curve * (1 - smooth_t) + product_energy * smooth_t
+
+# Vectorized tail flattening
+base_curve = np.where(reaction_coord < 1.5, reactant_energy, base_curve)
+base_curve = np.where(reaction_coord > 8.5, product_energy, base_curve)
+
+# Smooth join regions with repeated convolution
+kernel = np.ones(17) / 17
+energy_curve = base_curve.copy()
+for _ in range(3):
+    padded = np.pad(energy_curve, 17, mode="edge")
+    energy_curve = np.convolve(padded, kernel, mode="same")[17:-17]
+
+curve_points = list(zip(reaction_coord.tolist(), energy_curve.tolist(), strict=True))
+
+# Colorblind-safe palette
+BLUE = "#306998"
+DARK_BLUE = "#1A4971"
+TEAL = "#2980B9"
+ORANGE = "#E67E22"
+GRAY = "#AAAAAA"
+
+# Style — y-guides at key energies serve as reference lines
+custom_style = Style(
+    background="white",
+    plot_background="#FAFBFC",
+    foreground="#2C3E50",
+    foreground_strong="#2C3E50",
+    foreground_subtle="#CCCCCC",
+    colors=(BLUE, GRAY, GRAY, TEAL, ORANGE, DARK_BLUE, BLUE, BLUE),
+    title_font_size=60,
+    label_font_size=40,
+    major_label_font_size=36,
+    legend_font_size=32,
+    value_font_size=28,
+    stroke_width=5,
+)
+
+# Chart
+chart = pygal.XY(
+    width=4800,
+    height=2700,
+    style=custom_style,
+    title="line-reaction-coordinate \u00b7 pygal \u00b7 pyplots.ai",
+    x_title="Reaction Coordinate",
+    y_title="Potential Energy (kJ/mol)",
+    show_legend=True,
+    legend_at_bottom=True,
+    legend_at_bottom_columns=5,
+    show_x_guides=False,
+    show_y_guides=True,
+    show_x_labels=False,
+    dots_size=0,
+    stroke=True,
+    margin=80,
+    margin_left=260,
+    margin_right=160,
+    margin_bottom=200,
+    margin_top=120,
+    range=(8, 140),
+    xrange=(-0.2, 10.2),
+    truncate_legend=-1,
+    tooltip_border_radius=8,
+)
+
+# Main energy curve
+chart.add("Energy Profile", curve_points, stroke_style={"width": 6}, show_dots=False, fill=False)
+
+# Horizontal reference lines at reactant and product energy levels
+chart.add(
+    None,
+    [(0.0, reactant_energy), (10.0, reactant_energy)],
+    stroke_style={"width": 3, "dasharray": "16, 8"},
+    show_dots=False,
+)
+chart.add(
+    None,
+    [(0.0, product_energy), (10.0, product_energy)],
+    stroke_style={"width": 3, "dasharray": "16, 8"},
+    show_dots=False,
+)
+
+# Ea vertical indicator at transition state peak (teal — colorblind-safe)
+ea_x = peak_pos
+chart.add(
+    f"Ea = {activation_energy:.0f} kJ/mol",
+    [{"value": (ea_x, reactant_energy), "node": {"r": 14}}, {"value": (ea_x, transition_energy), "node": {"r": 14}}],
+    stroke_style={"width": 5, "dasharray": "8, 5"},
+)
+
+# ΔH vertical indicator (orange — colorblind-safe)
+dh_x = 8.5
+chart.add(
+    f"\u0394H = {enthalpy_change:.0f} kJ/mol",
+    [{"value": (dh_x, reactant_energy), "node": {"r": 14}}, {"value": (dh_x, product_energy), "node": {"r": 14}}],
+    stroke_style={"width": 5, "dasharray": "8, 5"},
+)
+
+# Transition state marker (larger, dark blue)
+chart.add(
+    "Transition State (\u2021)",
+    [{"value": (peak_pos, transition_energy), "node": {"r": 22}}],
+    stroke_style={"width": 0},
+    dots_size=22,
+)
+
+# Reactant marker
+chart.add(
+    f"Reactants ({reactant_energy:.0f} kJ/mol)",
+    [{"value": (1.0, reactant_energy), "node": {"r": 16}}],
+    stroke_style={"width": 0},
+    dots_size=16,
+)
+
+# Product marker
+chart.add(
+    f"Products ({product_energy:.0f} kJ/mol)",
+    [{"value": (9.5, product_energy), "node": {"r": 16}}],
+    stroke_style={"width": 0},
+    dots_size=16,
+)
+
+# Custom y-axis labels at chemically meaningful values
+chart.y_labels = [
+    {"label": f"{product_energy:.0f}", "value": product_energy},
+    {"label": f"{reactant_energy:.0f}", "value": reactant_energy},
+    {"label": "80", "value": 80},
+    {"label": "100", "value": 100},
+    {"label": f"{transition_energy:.0f}", "value": transition_energy},
+]
+
+# Save
+chart.render_to_file("plot.html")
+chart.render_to_png("plot.png")
diff --git a/plots/line-reaction-coordinate/metadata/pygal.yaml b/plots/line-reaction-coordinate/metadata/pygal.yaml
new file mode 100644
index 0000000000..d534073577
--- /dev/null
+++ b/plots/line-reaction-coordinate/metadata/pygal.yaml
@@ -0,0 +1,234 @@
+library: pygal
+specification_id: line-reaction-coordinate
+created: '2026-03-21T20:46:01Z'
+updated: '2026-03-21T21:18:10Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 23388404498
+issue: 4409
+python_version: 3.14.3
+library_version: 3.1.0
+preview_url: https://storage.googleapis.com/pyplots-images/plots/line-reaction-coordinate/pygal/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/line-reaction-coordinate/pygal/plot_thumb.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/line-reaction-coordinate/pygal/plot.html
+quality_score: 85
+review:
+  strengths:
+  - Excellent data quality with realistic chemistry context, proper units, and scientifically
+    accurate values
+  - Clean, well-structured deterministic code with vectorized numpy curve generation
+  - Good use of pygal-specific features (node customization, stroke_style dasharray,
+    HTML export)
+  - Custom y-labels at chemically meaningful energy values (20, 50, 80, 100, 120)
+  - Colorblind-safe palette with clear visual hierarchy and color-coded indicators
+  weaknesses:
+  - Labels for reactants/products/transition state appear in legend only, not directly
+    on the plot (pygal limitation)
+  - Ea and ΔH indicators are dashed lines rather than double-headed arrows (pygal
+    limitation)
+  - Some wasted whitespace in upper plot area above the 120 kJ/mol mark
+  image_description: 'The plot shows a reaction coordinate energy diagram rendered
+    as a pygal XY chart on a white background with a light gray (#FAFBFC) plot area.
+    A smooth blue curve traces the energy profile from left to right, starting flat
+    at ~50 kJ/mol (reactant level), rising steeply to a peak at ~120 kJ/mol (transition
+    state), then descending smoothly to ~20 kJ/mol (product level). Two horizontal
+    dashed gray lines span the full width at the reactant (50) and product (20) energy
+    levels, providing reference. A vertical dashed teal line connects the reactant
+    energy level to the transition state peak, representing Ea = 70 kJ/mol, with dots
+    at both endpoints. A vertical dashed orange line on the right side connects the
+    reactant and product energy levels, representing ΔH = -30 kJ/mol, also with endpoint
+    dots. A large dark blue dot marks the transition state near the curve peak. Smaller
+    blue dots mark the reactant (left, 50 kJ/mol) and product (right, 20 kJ/mol) positions.
+    The y-axis shows "Potential Energy (kJ/mol)" with custom labels at 20, 50, 80,
+    100, 120. The x-axis shows "Reaction Coordinate" with no numeric labels. The title
+    reads "line-reaction-coordinate · pygal · pyplots.ai". A legend at the bottom
+    lists all series: Energy Profile, Ea = 70 kJ/mol, ΔH = -30 kJ/mol, Transition
+    State (‡), Reactants (50 kJ/mol), Products (20 kJ/mol).'
+  criteria_checklist:
+    visual_quality:
+      score: 27
+      max: 30
+      items:
+      - id: VQ-01
+        name: Text Legibility
+        score: 7
+        max: 8
+        passed: true
+        comment: Font sizes explicitly set (title=60, label=40, major_label=36, legend=32,
+          value=28). All text readable. Legend text slightly small relative to chart
+          area.
+      - id: VQ-02
+        name: No Overlap
+        score: 6
+        max: 6
+        passed: true
+        comment: No overlapping elements. Legend items well-spaced at bottom.
+      - id: VQ-03
+        name: Element Visibility
+        score: 5
+        max: 6
+        passed: true
+        comment: Main curve clearly visible with stroke_width=6. Transition state
+          marker prominent. Ea dashed line somewhat subtle against curve.
+      - id: VQ-04
+        name: Color Accessibility
+        score: 4
+        max: 4
+        passed: true
+        comment: Blue/teal/orange palette is colorblind-safe with good contrast.
+      - id: VQ-05
+        name: Layout & Canvas
+        score: 3
+        max: 4
+        passed: true
+        comment: Plot fills reasonable portion of canvas with explicit margins. Some
+          wasted whitespace above 120 mark.
+      - id: VQ-06
+        name: Axis Labels & Title
+        score: 2
+        max: 2
+        passed: true
+        comment: Y-axis has units (kJ/mol). X-axis appropriate for arbitrary reaction
+          progress.
+    design_excellence:
+      score: 13
+      max: 20
+      items:
+      - id: DE-01
+        name: Aesthetic Sophistication
+        score: 5
+        max: 8
+        passed: true
+        comment: Custom color palette, light gray plot background, explicitly styled
+          fonts. Above defaults but not publication-quality.
+      - id: DE-02
+        name: Visual Refinement
+        score: 4
+        max: 6
+        passed: true
+        comment: X-axis labels hidden, subtle grid via foreground_subtle, custom y-labels
+          at chemically meaningful values.
+      - id: DE-03
+        name: Data Storytelling
+        score: 4
+        max: 6
+        passed: true
+        comment: Clear visual hierarchy with large transition state marker as focal
+          point, color-coded Ea and ΔH indicators.
+    spec_compliance:
+      score: 14
+      max: 15
+      items:
+      - id: SC-01
+        name: Plot Type
+        score: 5
+        max: 5
+        passed: true
+        comment: Correct reaction coordinate energy diagram with smooth curve and
+          clear maximum.
+      - id: SC-02
+        name: Required Features
+        score: 3
+        max: 4
+        passed: true
+        comment: All key features present. Labels in legend rather than directly on
+          plot; lines rather than double-headed arrows (pygal limitation).
+      - id: SC-03
+        name: Data Mapping
+        score: 3
+        max: 3
+        passed: true
+        comment: X = reaction coordinate, Y = potential energy. Full data range shown.
+      - id: SC-04
+        name: Title & Legend
+        score: 3
+        max: 3
+        passed: true
+        comment: Correct title format. Descriptive legend labels with energy values.
+    data_quality:
+      score: 14
+      max: 15
+      items:
+      - id: DQ-01
+        name: Feature Coverage
+        score: 5
+        max: 6
+        passed: true
+        comment: Shows single-step exothermic reaction with reactants, transition
+          state, products, Ea, and ΔH.
+      - id: DQ-02
+        name: Realistic Context
+        score: 5
+        max: 5
+        passed: true
+        comment: Chemistry reaction coordinate with proper kJ/mol units. Scientifically
+          accurate.
+      - id: DQ-03
+        name: Appropriate Scale
+        score: 4
+        max: 4
+        passed: true
+        comment: Values (50, 120, 20 kJ/mol) realistic for typical exothermic reaction.
+    code_quality:
+      score: 10
+      max: 10
+      items:
+      - id: CQ-01
+        name: KISS Structure
+        score: 3
+        max: 3
+        passed: true
+        comment: Clean imports → data → curve → style → chart → save flow. No functions/classes.
+      - id: CQ-02
+        name: Reproducibility
+        score: 2
+        max: 2
+        passed: true
+        comment: Fully deterministic data generation.
+      - id: CQ-03
+        name: Clean Imports
+        score: 2
+        max: 2
+        passed: true
+        comment: 'All imports used: numpy, pygal, Style.'
+      - id: CQ-04
+        name: Code Elegance
+        score: 2
+        max: 2
+        passed: true
+        comment: Clean, well-organized with vectorized operations. Smoothing approach
+          appropriate.
+      - id: CQ-05
+        name: Output & API
+        score: 1
+        max: 1
+        passed: true
+        comment: Saves as plot.png and plot.html. Current pygal API.
+    library_mastery:
+      score: 7
+      max: 10
+      items:
+      - id: LM-01
+        name: Idiomatic Usage
+        score: 4
+        max: 5
+        passed: true
+        comment: Good pygal.XY usage with Style, stroke_style dasharray, custom node
+          sizes, legend_at_bottom_columns.
+      - id: LM-02
+        name: Distinctive Features
+        score: 3
+        max: 5
+        passed: true
+        comment: 'Uses pygal-specific: SVG-native with HTML export, node dict customization,
+          stroke_style dasharray, tooltip_border_radius.'
+  verdict: REJECTED
+impl_tags:
+  dependencies: []
+  techniques:
+  - manual-ticks
+  - html-export
+  patterns:
+  - data-generation
+  dataprep: []
+  styling:
+  - grid-styling