When selecting an image & two corresponding shapes (e.g. two circles drawn on the picture using the Measurement plugin) using the "+" Button at the top, there is no histogram shown. When selecting the same nodes via the "Input data" tab in the plugin, instead, the histogram of the whole image (i.e. not in respect to the selected shapes) is shown.
This behavior should be unified, probably in the first manner, by not showing the histogram if multiple ROIs are specified.