FIG. 3.

Cluster evolution dendrograms, defined in Section II B for (a) cases and (b) deaths. These apply hierarchical clustering to the distance $d(s,t)$ between adjacency matrices ${\text{Adj}}^{(t)}$ at varying times $t$, thereby grouping different dates according to the cluster structures at these times. The $y$-axis excludes the first 50 days for cases and 66 days for deaths, as the cluster structure of counts is trivial before these periods, respectively. Each cluster is an unbroken interval of dates. There is a clear break in the cluster structure between 03/01 and 03/02 for cases, and 03/18 and 03/19 for deaths, with a 17-day difference.