Here are a few quick heatmaps of air traffic between states in the US. According to the file metadata I made them in June 2013, but I can no longer recall why. Data are available here.
"Send me your data - PDF is fine," said no one ever
The public health paradox ("When public health works, it's invisible")
Let's make data a civic right
Scholarly impact of open access journals
Six months later, disease detectives still battling fungal meningitis outbreak