spec: add heatmap-adjacency specification

[github-actions]

New Specification: heatmap-adjacency

Related to #5240

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specification.md

heatmap-adjacency: Network Adjacency Matrix Heatmap

Description

A matrix-based representation of a network or graph where rows and columns represent nodes and cell color indicates the presence or weight of edges between them. This visualization complements node-link diagrams by excelling at revealing clusters, structural patterns, and density in large or dense networks where node-link layouts become cluttered. Reordering nodes by cluster, degree, or community membership exposes block-diagonal structure and makes group boundaries immediately visible.

Applications

• Visualizing social network connections to detect community structure and cliques
• Displaying brain connectivity matrices in neuroscience to identify functionally linked regions
• Analyzing bilateral trade relationships between countries to reveal trading blocs
• Showing co-occurrence patterns in text analysis to find related terms or topics

Data

• source (categorical) — source node identifier
• target (categorical) — target node identifier
• weight (float, optional) — edge weight or connection strength; defaults to 1 (binary presence) when absent
• Size: 20–200 nodes (adjacency matrix will be nodes × nodes)
• Example: A social network of 50 people with friendship strength scores

Notes

• Nodes should be reorderable by cluster, degree, or community assignment to expose block-diagonal structure
• For undirected graphs the matrix is symmetric; implementations should fill both triangles
• Color intensity maps to edge weight; absent edges should use a distinct background (e.g., white or near-white)
• Optional dendrogram along axes to show hierarchical clustering of node ordering
• Include a colorbar legend showing the weight scale
• Axis tick labels should display node names; for large networks consider showing only group boundaries

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