The term "Heavy" (for wake separation) in the ICAO context is 100% based on MTOW! In the context of this graph, these are flights where we carry 3 or 4 pilots, and I am not in the seat for takeoff or landing. We still operate at the control during the middle of the flight when the other pilots are on their rest break. Not sure where the name "Heavy" came from here, but is it just the term used at my airline (and probably others? Some use "relief crew")

Good call on the data smoothing - I will look into a fix for this!