This example renders a 20-node adjacency matrix as an SVG heatmap, using D3 v7 to visualize connections from the Facebook Pages-food network dataset. Rows and columns represent sorted nodes, with steelblue cells indicating an edge between the corresponding source and target nodes. The code uses `d3.csv`, `d3.csvParseRows`, and `d3.cross` to populate a binary matrix from node and edge CSV files, then draws each cell as a `rect` with a mouseover tooltip that displays row and column labels along with the edge value.