It appears the Turning Circle chart is poorly presented. It's not an X-axis/Y-axis graph at all that I can see. It's simply drawing the turning circle arcs showing the relative diameters; all at "000 ft" which I take to be sea level.
Secondly, have a look at the relative speed charts between the Hurricane I, 109 E3, and the Spitfire I. The speed curve for the Spitfire I looks remarkably like the one for the much-decried "Über Sissyfire IIa" in the current retail version 1.05, as does the curve for the E3 to the CoD v. 1.05 E3, and, by gosh.....there's JTDawg's old Rotol!!!!!
Obviously that chart is wrong and the author sadly misinformed.