I have a rectangular grid describing a surface and given in columns of [X,Y,Z]. When I scatter-plot X vs. Y I realize that points are missing. Is there an efficient way of finding out which points these are (and then add them with Z=NaN). I realize I can loop through the entire set of points and look for all combinations of X and Y within the grid and flag the ones not existing in the list, but I was hoping for a more "vectorized" way. Possibly some logical operation between my grid and a complete one? Don't know quite how to go about that.