1. A method of material separation in dual energy computed tomography (CT), comprising the steps of: a) acquiring dual energy data for image data as a subject portion is being scanned; b) plotting image data points in a vector plot based upon the dual energy data, the vector plot having HU values in both axes; c) defining in the vector plot a region of interest according to a common rule for a given one of the image data points; d) counting a number of the image data points inside the region of interest in the vector plot to define a density value for the given one of the image data points; e) repeating said steps c) and d) for each of the image data points in the vector plot; and f) plotting a density plot based upon the density values, the density plot having HU values in both axes.