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.
centerPoint_x = min(xlim) + range(xlim)/2;
quad1 = x >= centerPoint_x & y >= centerPoint_y;
quad2 = x < centerPOint_x & y >= centerPoint_y;