spec: add area-elevation-profile specification (#4603)

## New Specification: `area-elevation-profile`

Related to #4578

---

### specification.md

# area-elevation-profile: Terrain Elevation Profile Along Transect

## Description

A cross-sectional line plot showing ground elevation along a path or
transect line, with the area below the profile filled to create a
terrain silhouette. Distance along the transect is plotted on the x-axis
and elevation on the y-axis. This visualization is widely used in hiking
and cycling route planning, civil engineering corridor design, and
geomorphology to convey terrain shape and difficulty at a glance.

## Applications

- Hiking and trail planning: visualizing cumulative ascent, descent, and
route difficulty along a trail
- Civil engineering: evaluating elevation changes along a proposed road
or railway corridor
- Cycling stage profiles: showing climb categories and sprint points
along a race stage (Tour de France style)
- GIS and geomorphology: extracting and analyzing terrain cross-sections
from digital elevation models (DEMs)

## Data

- `distance` (numeric) - cumulative horizontal distance along the
transect in km or miles
- `elevation` (numeric) - ground elevation at each sample point in
meters or feet
- `landmark_name` (string, optional) - name of notable feature at that
point (e.g., summit, pass, town)
- `landmark_distance` (numeric, optional) - distance value where the
landmark is located
- `landmark_elevation` (numeric, optional) - elevation value where the
landmark is located
- Size: 50-500 sample points along the transect
- Example: a 120 km hiking trail with elevation sampled every 250 m,
featuring 6-8 labeled landmarks (summits, passes, huts)

## Notes

- The area below the profile line should be filled with a solid or
gradient fill to create the terrain silhouette effect
- Label start and end points with location names and elevations
- Annotate key landmarks (summits, passes, river crossings) with
vertical marker lines and text labels
- Use a vertical exaggeration factor (e.g., 10x) and note it on the plot
to make terrain features visible
- Optional: apply gradient coloring to the profile line or fill based on
slope steepness (green for flat, red for steep)
- The x-axis should show distance units, y-axis should show elevation
units with appropriate tick spacing

---

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---
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plots/area-elevation-profile/specification.md

@@ -0,0 +1,31 @@
+# area-elevation-profile: Terrain Elevation Profile Along Transect
+
+## Description
+
+A cross-sectional line plot showing ground elevation along a path or transect line, with the area below the profile filled to create a terrain silhouette. Distance along the transect is plotted on the x-axis and elevation on the y-axis. This visualization is widely used in hiking and cycling route planning, civil engineering corridor design, and geomorphology to convey terrain shape and difficulty at a glance.
+
+## Applications
+
+- Hiking and trail planning: visualizing cumulative ascent, descent, and route difficulty along a trail
+- Civil engineering: evaluating elevation changes along a proposed road or railway corridor
+- Cycling stage profiles: showing climb categories and sprint points along a race stage (Tour de France style)
+- GIS and geomorphology: extracting and analyzing terrain cross-sections from digital elevation models (DEMs)
+
+## Data
+
+- `distance` (numeric) - cumulative horizontal distance along the transect in km or miles
+- `elevation` (numeric) - ground elevation at each sample point in meters or feet
+- `landmark_name` (string, optional) - name of notable feature at that point (e.g., summit, pass, town)
+- `landmark_distance` (numeric, optional) - distance value where the landmark is located
+- `landmark_elevation` (numeric, optional) - elevation value where the landmark is located
+- Size: 50-500 sample points along the transect
+- Example: a 120 km hiking trail with elevation sampled every 250 m, featuring 6-8 labeled landmarks (summits, passes, huts)
+
+## Notes
+
+- The area below the profile line should be filled with a solid or gradient fill to create the terrain silhouette effect
+- Label start and end points with location names and elevations
+- Annotate key landmarks (summits, passes, river crossings) with vertical marker lines and text labels
+- Use a vertical exaggeration factor (e.g., 10x) and note it on the plot to make terrain features visible
+- Optional: apply gradient coloring to the profile line or fill based on slope steepness (green for flat, red for steep)
+- The x-axis should show distance units, y-axis should show elevation units with appropriate tick spacing

plots/area-elevation-profile/specification.yaml

@@ -0,0 +1,30 @@
+# Specification-level metadata for area-elevation-profile
+# Auto-synced to PostgreSQL on push to main
+
+spec_id: area-elevation-profile
+title: Terrain Elevation Profile Along Transect
+
+# Specification tracking
+created: "2026-03-06T19:53:43Z"
+updated: null
+issue: 4578
+suggested: MarkusNeusinger
+
+# Classification tags (applies to all library implementations)
+# See docs/reference/tagging-system.md for detailed guidelines
+tags:
+  plot_type:
+    - area
+    - line
+  data_type:
+    - numeric
+    - continuous
+    - spatial
+  domain:
+    - science
+    - engineering
+    - environment
+  features:
+    - annotated
+    - color-mapped
+    - comparison