It appears that you are combining your 2 points, 1 for each plot, and then adding them to the end of a single array that makes up the x-y graph. Each iteration of your loop, 2 points are added to a single plot, 1 point from the larger random number and 1 point from the smaller random number.
You may want to consider a shift register for each array where you append the new points to their respective arrays, then combine the 2 arrays into 1 larger multi-dimensional array that gets fed into the XY graph.
