Plotting a histogram of particle intensity vs pixel
The expected distribution of intensity versus x,y position is formed by two idependent and identically distributed gaussian random variables; one for each axis. The distribution shown is for one "slice" through this 2D distribution.  The top view of the distribution can be envisioned as a 3d Gaussian "hill", and the histogram shown is for one slice through this hill as viewed from the side, where the slice can be from any direction as long as it goes through the center (origin).