spec: add dendrogram-radial specification (#5256)

## New Specification: `dendrogram-radial`

Related to #5245

---

### specification.md

# dendrogram-radial: Radial Dendrogram

## Description

A radial dendrogram renders hierarchical clustering in a circular layout
where the root node sits at the center and branches extend outward, with
leaf nodes arranged around the circumference. This layout is a
space-efficient alternative to linear dendrograms for large hierarchies,
making it well-suited for datasets with hundreds of leaves. Branch
lengths are proportional to distance or dissimilarity, preserving the
quantitative interpretation of cluster merges.

## Applications

- Displaying phylogenetic trees with hundreds of species in a compact
circular form
- Visualizing organizational hierarchies or reporting structures without
excessive horizontal/vertical scrolling
- Showing file system or taxonomy structures where many leaf nodes need
to be visible simultaneously
- Presenting clustering dendrograms for gene expression data with
color-coded cluster assignments

## Data

- `linkage_matrix` (numeric matrix) - hierarchical clustering linkage
matrix in scipy format (n-1 × 4), encoding merge indices, distances, and
cluster sizes
- `labels` (string[]) - names for each leaf node, displayed around the
circumference
- `cluster_colors` (string[] or int[], optional) - cluster assignment
for each leaf, used to color branches or an outer metadata ring
- Size: 20-500 leaf nodes recommended; radial layout excels where linear
dendrograms become unwieldy

## Notes

- Root is positioned at the center; leaves are placed at equal angular
spacing around the circumference
- Branch length (radial distance) should be proportional to merge
distance/dissimilarity
- Color branches by cluster assignment using a categorical colormap
- Optional: add a color-coded ring around the outer edge to encode
additional metadata (e.g., species family, department)
- Use scipy.cluster.hierarchy for linkage computation; polar projection
in matplotlib or dedicated libraries for radial rendering
- Consider adding interactive tooltips for leaf labels when the number
of leaves exceeds readable text size

---

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---
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plots/dendrogram-radial/specification.md

@@ -0,0 +1,28 @@
+# dendrogram-radial: Radial Dendrogram
+
+## Description
+
+A radial dendrogram renders hierarchical clustering in a circular layout where the root node sits at the center and branches extend outward, with leaf nodes arranged around the circumference. This layout is a space-efficient alternative to linear dendrograms for large hierarchies, making it well-suited for datasets with hundreds of leaves. Branch lengths are proportional to distance or dissimilarity, preserving the quantitative interpretation of cluster merges.
+
+## Applications
+
+- Displaying phylogenetic trees with hundreds of species in a compact circular form
+- Visualizing organizational hierarchies or reporting structures without excessive horizontal/vertical scrolling
+- Showing file system or taxonomy structures where many leaf nodes need to be visible simultaneously
+- Presenting clustering dendrograms for gene expression data with color-coded cluster assignments
+
+## Data
+
+- `linkage_matrix` (numeric matrix) - hierarchical clustering linkage matrix in scipy format (n-1 × 4), encoding merge indices, distances, and cluster sizes
+- `labels` (string[]) - names for each leaf node, displayed around the circumference
+- `cluster_colors` (string[] or int[], optional) - cluster assignment for each leaf, used to color branches or an outer metadata ring
+- Size: 20-500 leaf nodes recommended; radial layout excels where linear dendrograms become unwieldy
+
+## Notes
+
+- Root is positioned at the center; leaves are placed at equal angular spacing around the circumference
+- Branch length (radial distance) should be proportional to merge distance/dissimilarity
+- Color branches by cluster assignment using a categorical colormap
+- Optional: add a color-coded ring around the outer edge to encode additional metadata (e.g., species family, department)
+- Use scipy.cluster.hierarchy for linkage computation; polar projection in matplotlib or dedicated libraries for radial rendering
+- Consider adding interactive tooltips for leaf labels when the number of leaves exceeds readable text size

plots/dendrogram-radial/specification.yaml

@@ -0,0 +1,27 @@
+# Specification-level metadata for dendrogram-radial
+# Auto-synced to PostgreSQL on push to main
+
+spec_id: dendrogram-radial
+title: Radial Dendrogram
+
+# Specification tracking
+created: "2026-04-11T20:23:06Z"
+updated: null
+issue: 5245
+suggested: MarkusNeusinger
+
+# Classification tags (applies to all library implementations)
+# See docs/reference/tagging-system.md for detailed guidelines
+tags:
+  plot_type:
+    - dendrogram
+  data_type:
+    - hierarchical
+    - numeric
+  domain:
+    - science
+    - general
+  features:
+    - radial
+    - color-mapped
+    - clustering