The Hot/Crazy Scale is a graph used to display someone's hot-to-crazy ratio. As Barney explains in How I Met Everyone Else, a person is allowed to be crazy, as long as they are equally hot. Ideally, you want your date to be above the diagonal line, indicating that they are hotter than they are crazy. There does appear to be some bias however, as shown by Barney's diagram, which indicates that someone who is the maximum level of crazy does not have to be the maximum level of hot in order to be equal to/above the diagonal line.