See also http://www.mathworks.com/matlabcentral/fileexchange/19280-bivariant-kernel-density-estimation-v2-0/content/html/gkde2test.html http://www.mathworks.com

See also http://www.mathworks.com/matlabcentral/fileexchange/19280-bivariant-kernel-density-estimation-v2-0/content/html/gkde2test.html http://www.mathworks.com/matlabcentral/fileexchange/19280-bi... amongst other density estimators, for a nice smooth plot of the estimated joint density. (Downside being there's still a somewhat-arbitrary choice to be made for the smoothing bandwidth, and it's not quite as visually obvious as the choice of number of bins in a histogram plot.)