Data density exceeds Plot Pixel Density

I am using imagesc to display a large 2-D dataset as a color map. Since this dataset is large, what I notice is that there are data points which are not always displayed in the color map, since the number of pixels available to the colormap's screen real estate is smaller than the number of data points. Obviously, Matlab is performing some downsampling in the background to make the data fit the plot, but I do not know the algorithm it uses to do this. What I would like to do is determine what data points from the 2-D data set are not displayed in the final plot on my monitor. Even just getting some more basic understanding of how Matlab handles this problem would be helpful to me if known.