Yea well that's just too individual isn't it? It depends so much on the vehicle and the wheels and their air pressure in that special moment, it depends on the wind and of the shape/position of the rider and her weight and the grounds friction etc. etc.
The curve can be derived using the calculus of variations and it is indeed the best one in terms of minimizing the time taken to at from the start to the finish. The original image is on Wikipedia and is in the public domain but I think the author deserves a credit: Robert Ferreol.