Adding custom seasonalities

So far, the only seasonalities we have worked with are the defaults in Prophet: yearly, weekly, and daily. But there is no reason to limit ourselves to these seasonalities. If your data contains a cycle that is either longer or shorter than the 365.25-day yearly cycle, the 7-day weekly cycle, or the 1-day daily cycle, Prophet makes it easy to model this seasonality yourself.

A great example of a non-standard seasonality is the 11-year cycle of sunspots. Sunspots are regions on the Sun's surface that temporarily exhibit a much-reduced temperature, and hence appear much darker, than surrounding areas.

Beginning in approximately 1609, Galileo Galilei began systematic observation of sunspots and over the last 400+ years, this phenomenon has been constantly recorded. In fact, sunspots represent the longest continuously recorded time series of any natural phenomenon. Through these observations, scientists have identified a quasi-periodic cycle of 11...