The lovely light green ellipses are solitaries not part of a cluster. There were a lot more clusters than distinguishable colors, so our apologies for the use of one color (like orange) to represent more than one cluster. We did experiment with using rectangles as well as ellipses - this was not a visual success. Also, some of the larger clusters have so many events there is no room to draw enough markers.
Our software roughly defines the clusters in terms of two latitudes and two longitudes. Drawing these rectangles was simple, but the resulting visual was intensely disliked by almost all viewers: besides the introduction of obvious artifacts, it was difficult to tell how many events were in the cluster.