Heatmap

A symmetric $9 \times 9$ matrix drawn as a heatmap, each cell shaded on a red-to-gold ramp by its value. The coloring makes the block structure immediate: rows a–d are strongly coupled to one another, while e–i carry large diagonal entries but almost no weight off it, so they are nearly decoupled from the rest. Posted as an answer to https://tex.stackexchange.com/q/44868.