According to FlowingData:
“…a comprehensive view of flight delays, but here’s the part I found most interesting. This is a view of flight cancellations. The more red a rectangle is, the higher the percentage of cancellations
So following 9/11 there were actually fewer cancellations (and delays). My best bet is that this was because there were so few people flying after that event.”
Thanks to the contrarian blog for this.