That has to be the most generalised and inaccurate diagram I have ever seen.
The chart is deliberately generalized. It was generated for an article written by Battle of Britain ace and later Squadron Leader, T.S. (Wimpy) Wade, published in The Aeroplane magazine. It was intended to give a general understanding of turn radii, rather than a precise figure. Wade would later be a Hawker test pilot, and died in a crash of the P.1081 (a precursor of the Hunter) in 1951.