I might have mentioned this before, but now I can illustrate. Below is the graph of my rank on the Go server I usually play on.
Notice anything unusual? Or perhaps I should say usual?
Although it's not obvious at first due to the irregularity of the graph, it appears to have a 6 month period. Peaks and Troughs are about 3 months away from each other, and the pattern repeats every six months. I should start saving screen captures of these rank graphs every year so I can have a record of how it changes over greater spans of time than one year.