feat(letsplot): implement line-stress-strain (#5131)

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

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

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

**Parent Issue:** #4413

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/23363004980)*

---------

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/letsplot.py

@@ -0,0 +1,240 @@
+""" pyplots.ai
+line-stress-strain: Engineering Stress-Strain Curve
+Library: letsplot 4.9.0 | Python 3.14.3
+Quality: 92/100 | Created: 2026-03-20
+"""
+
+import numpy as np
+import pandas as pd
+from lets_plot import *  # noqa: F403
+from lets_plot.export import ggsave as export_ggsave
+
+
+LetsPlot.setup_html()  # noqa: F405
+
+# Data - Mild steel tensile test simulation
+np.random.seed(42)
+
+# Material properties for mild steel
+youngs_modulus = 210000  # MPa
+yield_strength = 250  # MPa
+uts = 400  # MPa (ultimate tensile strength)
+fracture_strain = 0.35
+uts_strain = 0.22
+yield_strain = yield_strength / youngs_modulus  # ~0.00119
+
+# Elastic region (0 to yield)
+n_elastic = 60
+strain_elastic = np.linspace(0, yield_strain, n_elastic)
+stress_elastic = youngs_modulus * strain_elastic
+
+# Yield plateau (mild steel has a distinct yield point)
+n_plateau = 20
+strain_plateau = np.linspace(yield_strain, 0.015, n_plateau)
+stress_plateau = yield_strength + np.random.normal(0, 1.5, n_plateau)
+
+# Strain hardening region (from end of plateau to UTS)
+n_hardening = 120
+strain_hardening = np.linspace(0.015, uts_strain, n_hardening)
+stress_hardening = yield_strength + (uts - yield_strength) * (
+    1 - np.exp(-8 * (strain_hardening - 0.015) / (uts_strain - 0.015))
+)
+stress_hardening += np.random.normal(0, 1.0, n_hardening)
+
+# Necking region (UTS to fracture)
+n_necking = 60
+strain_necking = np.linspace(uts_strain, fracture_strain, n_necking)
+stress_necking = uts - (uts - 280) * ((strain_necking - uts_strain) / (fracture_strain - uts_strain)) ** 1.5
+stress_necking += np.random.normal(0, 1.5, n_necking)
+
+# Combine all regions
+strain = np.concatenate([strain_elastic, strain_plateau, strain_hardening, strain_necking])
+stress = np.concatenate([stress_elastic, stress_plateau, stress_hardening, stress_necking])
+
+df = pd.DataFrame({"strain": strain, "stress": stress})
+
+# 0.2% offset line for yield point determination
+offset_val = 0.002
+offset_line_strain = np.linspace(offset_val, offset_val + yield_strength / youngs_modulus + 0.003, 50)
+offset_line_stress = youngs_modulus * (offset_line_strain - offset_val)
+offset_line_stress = np.clip(offset_line_stress, 0, yield_strength + 30)
+df_offset = pd.DataFrame({"strain": offset_line_strain, "stress": offset_line_stress})
+
+# Key points
+yield_point_strain = offset_val + yield_strength / youngs_modulus
+yield_point_stress = yield_strength
+fracture_stress = stress_necking[-1]
+
+df_points = pd.DataFrame(
+    {
+        "strain": [yield_point_strain, uts_strain, fracture_strain],
+        "stress": [yield_point_stress, uts, fracture_stress],
+        "label": [f"Yield Point ({yield_strength} MPa)", f"UTS ({uts} MPa)", f"Fracture ({fracture_stress:.0f} MPa)"],
+        "type": ["Yield", "UTS", "Fracture"],
+    }
+)
+
+# Consolidated annotations DataFrame
+df_annotations = pd.DataFrame(
+    {
+        "x": [yield_point_strain + 0.012, uts_strain + 0.015, fracture_strain - 0.045, 0.008, 0.007, 0.005, 0.11, 0.29],
+        "y": [yield_point_stress + 15, uts + 10, fracture_stress - 30, 130, 60, 350, 350, 350],
+        "label": [
+            f"Yield Point\n({yield_strength} MPa)",
+            f"UTS ({uts} MPa)",
+            "Fracture",
+            f"E = {youngs_modulus // 1000} GPa",
+            "0.2% offset",
+            "Elastic",
+            "Strain Hardening",
+            "Necking",
+        ],
+        "group": ["yield", "uts", "fracture", "modulus", "offset", "region", "region", "region"],
+    }
+)
+
+# Colorblind-safe palette: blue, purple, gray (avoids orange/green pair)
+color_yield = "#9467BD"  # purple
+color_uts = "#D62728"  # red
+color_fracture = "#7F7F7F"  # gray
+color_main = "#306998"  # Python blue
+color_offset = "#E377C2"  # pink
+
+# Segment connector lines from key points to annotations (distinctive lets-plot feature)
+df_segments = pd.DataFrame(
+    {
+        "x": [yield_point_strain, uts_strain, fracture_strain],
+        "y": [yield_point_stress, uts, fracture_stress],
+        "xend": [yield_point_strain + 0.011, uts_strain + 0.014, fracture_strain - 0.035],
+        "yend": [yield_point_stress + 12, uts + 8, fracture_stress - 22],
+    }
+)
+
+# Plot
+plot = (
+    ggplot()
+    # Region background bands using geom_rect (distinctive lets-plot feature)
+    + geom_rect(
+        aes(xmin="xmin", xmax="xmax", ymin="ymin", ymax="ymax", fill="region"),
+        data=pd.DataFrame(
+            {
+                "xmin": [0, 0.015, uts_strain],
+                "xmax": [0.015, uts_strain, fracture_strain],
+                "ymin": [0, 0, 0],
+                "ymax": [460, 460, 460],
+                "region": ["Elastic", "Strain Hardening", "Necking"],
+            }
+        ),
+        alpha=0.35,
+    )
+    + scale_fill_manual(
+        values={
+            "Elastic": "#DAE8FC",
+            "Strain Hardening": "#FFF2CC",
+            "Necking": "#F8D7DA",
+            "Yield": color_yield,
+            "UTS": color_uts,
+            "Fracture": color_fracture,
+        }
+    )
+    # Main stress-strain curve with tooltips (distinctive lets-plot feature)
+    + geom_line(
+        aes(x="strain", y="stress"),
+        data=df,
+        color=color_main,
+        size=2.0,
+        tooltips=layer_tooltips()
+        .format("strain", ".4f")
+        .format("stress", ".1f")
+        .line("Strain: @strain")
+        .line("Stress: @stress MPa"),
+    )
+    # 0.2% offset line
+    + geom_line(aes(x="strain", y="stress"), data=df_offset, color=color_offset, size=1.2, linetype="dashed")
+    # Segment connectors from points to labels (geom_segment - distinctive feature)
+    + geom_segment(
+        aes(x="x", y="y", xend="xend", yend="yend"), data=df_segments, color="#999999", size=0.6, linetype="dotted"
+    )
+    # Key points with tooltips (distinctive lets-plot feature)
+    + geom_point(
+        aes(x="strain", y="stress", fill="type"),
+        data=df_points,
+        color="white",
+        size=7,
+        shape=21,
+        stroke=1.2,
+        tooltips=layer_tooltips().line("@type").line("Strain: @strain").line("Stress: @stress MPa"),
+    )
+    + guides(fill="none")
+    # Annotations - key points
+    + geom_text(
+        aes(x="x", y="y", label="label"),
+        data=df_annotations.query("group == 'yield'"),
+        size=11,
+        color=color_yield,
+        hjust=0,
+    )
+    + geom_text(
+        aes(x="x", y="y", label="label"), data=df_annotations.query("group == 'uts'"), size=11, color=color_uts, hjust=0
+    )
+    + geom_text(
+        aes(x="x", y="y", label="label"),
+        data=df_annotations.query("group == 'fracture'"),
+        size=11,
+        color=color_fracture,
+        hjust=0.5,
+    )
+    # Elastic modulus annotation
+    + geom_text(
+        aes(x="x", y="y", label="label"),
+        data=df_annotations.query("group == 'modulus'"),
+        size=10,
+        color=color_main,
+        hjust=0,
+        fontface="italic",
+    )
+    # Offset label
+    + geom_text(
+        aes(x="x", y="y", label="label"),
+        data=df_annotations.query("group == 'offset'"),
+        size=9,
+        color=color_offset,
+        hjust=0,
+        fontface="italic",
+    )
+    # Region labels
+    + geom_text(
+        aes(x="x", y="y", label="label"),
+        data=df_annotations.query("group == 'region'"),
+        size=12,
+        color="#666666",
+        fontface="italic",
+    )
+    # Styling
+    + labs(
+        x="Engineering Strain",
+        y="Engineering Stress (MPa)",
+        title="line-stress-strain \u00b7 letsplot \u00b7 pyplots.ai",
+    )
+    + scale_x_continuous(breaks=[0, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35])
+    + scale_y_continuous(breaks=[0, 50, 100, 150, 200, 250, 300, 350, 400, 450])
+    + ggsize(1600, 900)
+    + theme_minimal()
+    + theme(
+        axis_text=element_text(size=16, color="#555555"),
+        axis_title=element_text(size=20, color="#333333"),
+        plot_title=element_text(size=24, color="#222222", face="bold"),
+        panel_grid_major_x=element_blank(),
+        panel_grid_major_y=element_line(color="#E0E0E0", size=0.3),
+        panel_grid_minor=element_blank(),
+        plot_background=element_rect(fill="#FAFAFA", color="#FAFAFA"),
+        panel_background=element_rect(fill="transparent", color="transparent"),
+        axis_ticks=element_blank(),
+        axis_ticks_length=0,
+        plot_margin=[30, 40, 20, 20],
+    )
+)
+
+# Save
+export_ggsave(plot, filename="plot.png", path=".", scale=3)
+export_ggsave(plot, filename="plot.html", path=".")