I don't find the histogram function provides vey much resolution at all because the software automatically sets the scale. If you look at the gallery of installations on the VMSPc website and scroll down then you will see several examples of others who have setup the histogram function. I just don't see very much variation in those traces or ones I have played with on my setup.
You might try one value only such as RPM because it does vary a bit and see if you get a response on the software.