The concept is very cool, but I find the UI lacking. I'd expect to see a cumulative expansion from patient zero, with a more continuous timeline. This appears to show the current visitors at that moment in time, which could allow for the viewer to see how visits fluctuate with time of day and surges grow larger over time, but it fails to even show this very well.

Of course it's easy to criticize, so props for the neat idea and decent execution.