N=10

//Generate (useless) data
x=0:1:10;  // size: 1*11
y=0:1:20; // size: 1*21
z=ceil(N*rand(11,21)); //size: 11*21

//Plot
scf(); //Create a new grahic window
xset("colormap",graycolormap(N)); // Choose colormap (see the help of colormap)
xtitle("Data", "x coordinate", "y coordinate");
colorbar(min(z), max(z)); // Draw a colorbar
grayplot(x, y, z); // Actually plot them