If the photometry graph has no scatter, and the gradient graph is perfectly horizontal (no scatter), this means that the reference and target image you are comparing are identical. Somehow you have duplicated some of the images. Try selecting a different target image - you should then see the graphs you were expecting.... the photometry graph is the diagonal you expect but there are no outliers/spread from the diagonal. Not sure what this is telling me about the star selection within NSG + my frames. I guess that means that the frames are good / star selection choices available are good quality?
NSG is not designed to remove the gradient. Instead, it removes the relative gradient between the chosen reference and the target images. The integrated image's gradient should be very similar to the gradient in the reference frame. If you wish to minimize the gradient, you need to use the image with the least gradient as the reference.I did complete the process through image integration for the OSC image and compared it to the integrated image for the same mosaic pane created by WBPP. I was surprised that there was more visible gradient across the frame from the NSG image than the WBPP integration as I was not expecting that result.
The main advantage of using NSG is to improve ImageIntegration's data rejection, and to determine more accurate (brightness) scale and image weights. Note that all normalized image will have approximately the same gradient as the selected reference frame.