feat(seaborn): implement line-stress-strain (#5130)

## Implementation: `line-stress-strain` - seaborn

Implements the **seaborn** version of `line-stress-strain`.

**File:** `plots/line-stress-strain/implementations/seaborn.py`

**Parent Issue:** #4413

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/23363004930)*

---------

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-stress-strain/implementations/seaborn.py

@@ -0,0 +1,179 @@
+""" pyplots.ai
+line-stress-strain: Engineering Stress-Strain Curve
+Library: seaborn 0.13.2 | Python 3.14.3
+Quality: 90/100 | Created: 2026-03-20
+"""
+
+import matplotlib.pyplot as plt
+import numpy as np
+import pandas as pd
+import seaborn as sns
+
+
+# Data — Mild steel tensile test simulation
+np.random.seed(42)
+
+youngs_modulus = 200000  # MPa
+yield_stress = 250  # MPa
+uts = 400  # MPa
+fracture_strain = 0.35
+uts_strain = 0.22
+yield_strain = yield_stress / youngs_modulus
+
+# Elastic region
+strain_elastic = np.linspace(0, yield_strain, 60)
+stress_elastic = youngs_modulus * strain_elastic
+
+# Strain hardening (quadratic rise to UTS)
+strain_hardening = np.linspace(yield_strain, uts_strain, 200)
+t = (strain_hardening - yield_strain) / (uts_strain - yield_strain)
+stress_hardening = yield_stress + (uts - yield_stress) * (2 * t - t**2)
+
+# Necking region (UTS to fracture — stress drops)
+strain_necking = np.linspace(uts_strain, fracture_strain, 100)
+t_neck = (strain_necking - uts_strain) / (fracture_strain - uts_strain)
+fracture_stress = 320
+stress_necking = uts - (uts - fracture_stress) * t_neck**1.5
+
+# Build DataFrame with region labels for seaborn hue-based plotting
+df_elastic = pd.DataFrame({"strain": strain_elastic, "stress": stress_elastic, "region": "Elastic"})
+df_hardening = pd.DataFrame({"strain": strain_hardening, "stress": stress_hardening, "region": "Strain Hardening"})
+df_necking = pd.DataFrame({"strain": strain_necking, "stress": stress_necking, "region": "Necking"})
+df = pd.concat([df_elastic, df_hardening, df_necking], ignore_index=True)
+
+# 0.2% offset yield point
+offset = 0.002
+offset_line_strain = np.linspace(offset, offset + yield_stress / youngs_modulus + 0.003, 50)
+offset_line_stress = youngs_modulus * (offset_line_strain - offset)
+offset_line_stress = np.clip(offset_line_stress, 0, yield_stress + 20)
+
+yield_offset_strain = offset + yield_stress / youngs_modulus
+yield_offset_stress = yield_stress
+
+# Define seaborn palette and style
+region_palette = {"Elastic": "#306998", "Strain Hardening": "#E07B39", "Necking": "#C0392B"}
+sns.set_style("whitegrid", {"grid.linestyle": "-", "grid.alpha": 0.15, "grid.linewidth": 0.8})
+sns.set_context(
+    "talk",
+    rc={
+        "axes.titlesize": 24,
+        "axes.labelsize": 20,
+        "xtick.labelsize": 16,
+        "ytick.labelsize": 16,
+        "legend.fontsize": 13,
+        "font.weight": "medium",
+    },
+)
+
+fig, ax = plt.subplots(figsize=(16, 9))
+
+# Plot stress-strain curve using seaborn lineplot with hue for regions
+sns.lineplot(
+    data=df, x="strain", y="stress", hue="region", palette=region_palette, linewidth=3.5, ax=ax, legend=True, sort=False
+)
+
+# 0.2% offset line
+ax.plot(offset_line_strain, offset_line_stress, linestyle="--", linewidth=2, color="#808080", zorder=3)
+
+# Mark critical points using seaborn scatterplot for consistent styling
+critical_points = pd.DataFrame(
+    {
+        "strain": [yield_offset_strain, uts_strain, fracture_strain],
+        "stress": [yield_offset_stress, uts, fracture_stress],
+        "point": ["Yield Point", "UTS", "Fracture"],
+    }
+)
+point_palette = {"Yield Point": "#E07B39", "UTS": "#C0392B", "Fracture": "#2C3E50"}
+point_markers = {"Yield Point": "o", "UTS": "o", "Fracture": "X"}
+sns.scatterplot(
+    data=critical_points,
+    x="strain",
+    y="stress",
+    hue="point",
+    style="point",
+    palette=point_palette,
+    markers=point_markers,
+    s=200,
+    edgecolor="white",
+    linewidth=1.5,
+    ax=ax,
+    zorder=5,
+    legend=True,
+)
+
+# Annotations for critical points — yield moved right to reduce left-side clutter
+ax.annotate(
+    "Yield Point\n(0.2% offset)",
+    xy=(yield_offset_strain, yield_offset_stress),
+    xytext=(0.06, yield_offset_stress - 30),
+    fontsize=14,
+    fontweight="bold",
+    color="#E07B39",
+    arrowprops={"arrowstyle": "-|>", "color": "#E07B39", "lw": 1.5},
+    ha="left",
+    va="top",
+)
+
+ax.annotate(
+    f"UTS = {uts} MPa",
+    xy=(uts_strain, uts),
+    xytext=(uts_strain + 0.03, uts + 30),
+    fontsize=14,
+    fontweight="bold",
+    color="#C0392B",
+    arrowprops={"arrowstyle": "-|>", "color": "#C0392B", "lw": 1.5},
+    ha="left",
+    va="bottom",
+)
+
+ax.annotate(
+    "Fracture",
+    xy=(fracture_strain, fracture_stress),
+    xytext=(fracture_strain - 0.01, fracture_stress - 60),
+    fontsize=14,
+    fontweight="bold",
+    color="#2C3E50",
+    arrowprops={"arrowstyle": "-|>", "color": "#2C3E50", "lw": 1.5},
+    ha="center",
+    va="top",
+)
+
+# Elastic modulus annotation — positioned to the right to avoid left-side clutter
+ax.annotate(
+    f"E = {youngs_modulus // 1000} GPa",
+    xy=(yield_strain * 0.5, youngs_modulus * yield_strain * 0.5),
+    xytext=(0.05, 60),
+    fontsize=13,
+    fontstyle="italic",
+    color="#306998",
+    arrowprops={"arrowstyle": "-|>", "color": "#306998", "lw": 1.2},
+    ha="left",
+    va="center",
+)
+
+# Region shading using axvspan
+region_shading = [
+    (0, yield_strain, "#306998"),
+    (yield_strain, uts_strain, "#E07B39"),
+    (uts_strain, fracture_strain, "#C0392B"),
+]
+for x0, x1, color in region_shading:
+    ax.axvspan(x0, x1, alpha=0.06, color=color, zorder=0)
+
+# Style using seaborn's despine
+sns.despine(ax=ax)
+ax.xaxis.grid(False)
+ax.set_xlabel("Engineering Strain")
+ax.set_ylabel("Engineering Stress (MPa)")
+ax.set_title("line-stress-strain · seaborn · pyplots.ai", fontweight="medium")
+ax.set_xlim(-0.01, fracture_strain + 0.03)
+ax.set_ylim(-10, uts + 80)
+
+# Consolidate legend: combine region lines and critical point markers
+handles, labels = ax.get_legend_handles_labels()
+# Reorder: regions first, then critical points
+ax.legend(handles=handles, labels=labels, fontsize=13, loc="center right", framealpha=0.9, edgecolor="#cccccc")
+
+# Save
+plt.tight_layout()
+plt.savefig("plot.png", dpi=300, bbox_inches="tight")