Via placemap: Flow Map Layout, Phan et al., InfoVis'05.

The idea seems to be to visually express how much movement there is from different geographically placed sources to a single sink, by drawing the movement as a confluent tree in which physically nearby sources are merged together, and the thickness of a confluent edge indicates the amount of movement along that edge. So it's closely related to confluent graph drawing, but not the same, because the initial data is not so much a graph as a spatial numeric field (the amount of movement from each site) and the confluent tracks merge but don't unmerge. However, there is some graph drawing involved, in placing the edges and merge nodes of the resulting tree so that it doesn't cross itself.