Thanks for the graph; I think that is just normal behavior for the combination of the VNH5019 and your motor. Unfortunately, it is not practical for the VNH5019 to operate in drive-brake mode since the inputs required to do that are not capable of high-speed switching, so there is not much that can be done besides trying to compensate for the nonlinearity in software or using closed-loop feedback to control the speed. One thing you could try is playing around with the PWM frequency to see if that has any effect on linearity, and you could try applying some load to see if that changes anything.
-Jon