PIN number analysis
The first two digits are the X axis; the second two, the Y axis. The colour scaling is logarithmic.
The diagonal is repeated digits. The strong vertical line is numbers starting 19; in other words, years. The author has a somewhat more readable summary at The Guardian, with some interesting points:
In America, a driver’s licence, which everyone carries in their wallet with their ATM card, contains birthday information providing a thief with both the lock and key in the same location. If you have difficulty remembering a pin and elect to use a birth date, at least use that of your spouse.