This heatmap is created in d3js using https://github.com/erdogant/d3heatmap.
A network can be represented by an adjacency matrix, where each cell ij represents an edge from vertex i to vertex j.
Given this two-dimensional representation of a graph, a natural visualization is to show the matrix! However, the effectiveness of a matrix diagram is heavily dependent on the order of rows and columns: if related nodes are placed closed to each other, it is easier to identify clusters and bridges.
While path-following is harder in a matrix view than in a node-link diagram, matrices have other advantages. As networks get large and highly connected, node-link diagrams often devolve into giant hairballs of line crossings. Line crossings are impossible with matrix views. Matrix cells can also be encoded to show additional data; here color depicts clusters computed by a community-detection algorithm.