Common birthdays, you all ready knew that

Well this seems to be doing the rounds today on gizmodo, geekologie and buzzfeed, it's the how-common-is-your-birthday chart. Well guess what lots of people born 9 months after the new year!, hmm, suspicious much? Well given the data comes from the US it seems pretty likely being cooped up during winter may get people a bit frisky. It's data from 1973-1999.