Since the grid size is NxN, then wouldn't your X and Y vectors have N*N elements or coordinates each, one for each square in the grid? In your first example, your grid is a 4x4 matrix. What would the x coordinate be for each element in the grid? If we start at the bottom left and assume that this is 1, then
X = [1 2 3 4 4 3 2 1 ...];
where we move along the first (bottom) row from left to right, and assign an x-coordinate for each square. When we do the second row, we just move backwards, starting at the far right (4) and move to the left (1). We can continue in that pattern for each of the four rows.
With the Y coordinates, we do something similar. If we start at the bottom left and assume that this is 1, then
Y = [1 1 1 1 2 2 2 2 ...]
where we move along the first (bottom) row from left to right, and assign a y-coordinate for each square. Since y corresponds to row, all y values in each row are identical. When we move to the second row, the y coordinates just jump by one.
You can then plot the data as
plot(X,Y)
and see the different patterns.
