Presumably you already have the (x,y) coordinates if you've plotted the data.
You just need to use indexing (the main superpower of Matlab) to determine which points are in which quadrants.
For example,
centerPoint_x = min(xlim) + range(xlim)/2;
centerPoint_y = % Same thing as above but with ylim
quad1 = x >= centerPoint_x & y >= centerPoint_y;
quad2 = x < centerPOint_x & y >= centerPoint_y;
quad3 = % fill in....
quad4 = % file in....
% Points that are in Quad1 are
x(quad1), y(quad1)
