Toggling between the cropped and full image histograms would be a nice to have if it was easy to do.
The same could have been accomplished by removing the crop, however when you "Remove Crop" the crop goes away forever and the toggling between the two is not possible.
Nikon Capture has two histograms, one for the orignal image and one for the selected area. I often get confused at which on I'm looking at and try to stick to the selected area histogram to simplify my life.
I hope this helps.
\Marty