feat(letsplot): implement area-elevation-profile (#4894)

## Implementation: `area-elevation-profile` - letsplot

Implements the **letsplot** version of `area-elevation-profile`.

**File:** `plots/area-elevation-profile/implementations/letsplot.py`

**Parent Issue:** #4578

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/23120069378)*

---------

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/area-elevation-profile/implementations/letsplot.py

@@ -0,0 +1,175 @@
+""" pyplots.ai
+area-elevation-profile: Terrain Elevation Profile Along Transect
+Library: letsplot 4.9.0 | Python 3.14.3
+Quality: 85/100 | Created: 2026-03-15
+"""
+
+import numpy as np
+import pandas as pd
+from lets_plot import *  # noqa: F403, F401
+from lets_plot.export import ggsave as export_ggsave
+
+
+LetsPlot.setup_html()  # noqa: F405
+
+# Data — Alpine hiking trail elevation profile (120 km)
+np.random.seed(42)
+n_points = 480
+distance = np.linspace(0, 120, n_points)
+
+# Build realistic terrain with multiple overlapping features
+base_elevation = 1200
+broad_shape = 600 * np.sin(distance * np.pi / 120)
+ridge1 = 400 * np.exp(-((distance - 35) ** 2) / 80)
+ridge2 = 550 * np.exp(-((distance - 65) ** 2) / 120)
+ridge3 = 350 * np.exp(-((distance - 95) ** 2) / 60)
+valley = -300 * np.exp(-((distance - 50) ** 2) / 50)
+noise = np.cumsum(np.random.randn(n_points) * 3)
+elevation = base_elevation + broad_shape + ridge1 + ridge2 + ridge3 + valley + noise
+elevation = np.clip(elevation, 800, None)
+
+df = pd.DataFrame({"distance": distance, "elevation": elevation})
+
+# Landmarks along the trail
+landmark_names = [
+    "Talbach Village",
+    "Steinberg Pass",
+    "Grünsee Lake",
+    "Hochwand Summit",
+    "Felsentor Saddle",
+    "Alpenhof Hut",
+    "Gipfelkreuz Peak",
+    "Bergdorf Village",
+]
+landmark_distances = [0, 20, 38, 50, 65, 80, 95, 120]
+landmark_elevations = [float(np.interp(d, distance, elevation)) for d in landmark_distances]
+
+# Stagger label offsets — alternate high/low in crowded 50-100 km region
+# Alternating low/high nudge with horizontal shifts to prevent overlap
+nudge_y = [80, 250, 80, 250, 80, 250, 80, 80]
+nudge_x = [0, -2, 2, -3, 0, 0, 0, 0]
+landmarks_df = pd.DataFrame(
+    {
+        "distance": landmark_distances,
+        "elevation": landmark_elevations,
+        "name": landmark_names,
+        "label_y": [e + n for e, n in zip(landmark_elevations, nudge_y, strict=True)],
+        "label_x": [d + n for d, n in zip(landmark_distances, nudge_x, strict=True)],
+    }
+)
+
+# Compute slope for gradient coloring — smoothed to reduce visual fragmentation
+slope = np.gradient(elevation, distance)
+slope_abs = np.abs(slope)
+# Rolling average to prevent rapid color switching on steep transitions
+slope_smooth = pd.Series(slope_abs).rolling(window=25, center=True, min_periods=1).mean()
+slope_category = pd.cut(slope_smooth, bins=[0, 15, 40, np.inf], labels=["Flat/Gentle", "Moderate", "Steep"])
+df["slope_category"] = slope_category
+
+# Segment data for vertical landmark lines (using geom_segment)
+segments_df = pd.DataFrame(
+    {"x": landmark_distances, "y": landmark_elevations, "yend": [float(min(elevation)) - 20] * len(landmark_distances)}
+)
+
+# Colorblind-safe slope palette: blue, amber, deep purple
+slope_colors = ["#306998", "#E69F00", "#882255"]
+
+# Y-axis range — tighten floor to reduce dead space
+y_floor = int(min(elevation)) - 30
+y_max = int(max(elevation) * 1.15)
+
+# Plot
+plot = (
+    ggplot(df, aes(x="distance", y="elevation"))  # noqa: F405
+    # Filled area for terrain silhouette with gradient-like effect
+    + geom_area(fill="#306998", alpha=0.15)  # noqa: F405
+    + geom_area(fill="#306998", alpha=0.10)  # noqa: F405
+    # Profile line colored by slope steepness
+    + geom_line(  # noqa: F405
+        aes(color="slope_category"),  # noqa: F405
+        size=2.5,
+        tooltips=layer_tooltips()  # noqa: F405
+        .line("@|@elevation")
+        .line("Distance: @distance km")
+        .line("Slope: @slope_category"),
+    )
+    + scale_color_manual(  # noqa: F405
+        values=slope_colors, name="Slope Steepness"
+    )
+    # Vertical marker lines at landmarks using geom_segment
+    + geom_segment(  # noqa: F405
+        data=segments_df,
+        mapping=aes(x="x", y="yend", xend="x", yend="y"),  # noqa: F405
+        color="#AAAAAA",
+        size=0.6,
+        linetype="dashed",
+        inherit_aes=False,
+    )
+    # Landmark points — white filled circles with colored border
+    + geom_point(  # noqa: F405
+        data=landmarks_df,
+        mapping=aes(x="distance", y="elevation"),  # noqa: F405
+        size=7,
+        color="#306998",
+        fill="white",
+        shape=21,
+        stroke=2.5,
+        inherit_aes=False,
+        tooltips=layer_tooltips()  # noqa: F405
+        .line("@name")
+        .line("Elevation: @elevation m")
+        .line("Distance: @distance km"),
+    )
+    # Connector lines from labels to landmark points
+    + geom_segment(  # noqa: F405
+        data=landmarks_df,
+        mapping=aes(x="distance", y="elevation", xend="label_x", yend="label_y"),  # noqa: F405
+        color="#BBBBBB",
+        size=0.4,
+        linetype="dotted",
+        inherit_aes=False,
+    )
+    # Landmark labels — sized to match other text elements, well-staggered
+    + geom_text(  # noqa: F405
+        data=landmarks_df,
+        mapping=aes(x="label_x", y="label_y", label="name"),  # noqa: F405
+        size=14,
+        color="#2C3E50",
+        angle=40,
+        hjust=0,
+        fontface="bold",
+        inherit_aes=False,
+    )
+    # Scales and labels
+    + scale_x_continuous(  # noqa: F405
+        name="Distance (km)", breaks=list(range(0, 121, 20)), limits=[-2, 150]
+    )
+    + scale_y_continuous(  # noqa: F405
+        name="Elevation (m)", limits=[y_floor, y_max + 200], breaks=list(range(1000, y_max + 200, 200))
+    )
+    + labs(  # noqa: F405
+        title="Alpine Trail Elevation Profile · area-elevation-profile · letsplot · pyplots.ai",
+        subtitle="120 km hiking transect with 8 landmarks — vertical exaggeration ~10×",
+    )
+    + ggsize(1600, 900)  # noqa: F405
+    + theme_minimal()  # noqa: F405
+    + theme(  # noqa: F405
+        axis_text=element_text(size=16, color="#555555"),  # noqa: F405
+        axis_title=element_text(size=20, color="#333333"),  # noqa: F405
+        plot_title=element_text(size=24, color="#1A1A2E", face="bold"),  # noqa: F405
+        plot_subtitle=element_text(size=16, color="#666666"),  # noqa: F405
+        legend_text=element_text(size=14),  # noqa: F405
+        legend_title=element_text(size=16, face="bold"),  # noqa: F405
+        legend_position="bottom",
+        panel_grid_major_y=element_line(color="#E8E8E8", size=0.3),  # noqa: F405
+        panel_grid_major_x=element_blank(),  # noqa: F405
+        panel_grid_minor=element_blank(),  # noqa: F405
+        plot_margin=[40, 80, 20, 20],
+        plot_background=element_rect(fill="#FAFAFA", color="#FAFAFA"),  # noqa: F405
+        panel_background=element_rect(fill="#FAFAFA", color="#FAFAFA"),  # noqa: F405
+    )
+)
+
+# Save
+export_ggsave(plot, filename="plot.png", path=".", scale=3)
+export_ggsave(plot, filename="plot.html", path=".")