Looking at the values of V2 against weight for any given combination of temperature and altitude, I can see that there is a square root relationship. This might indicate that V2 is being determined by the stall speed. But presumably Vmu is also related to the square root of weight, so this might be the limiting factor.
If we take any given low weight and move left to right, the increasing temperature and altitude cause V2 to gradually reduce. But if we choose a high weight V2 initially remains constant then gradually reduces. Would this be the case if Vmu were the limiting factor?