Please see the MATLAB Wiki entry http://matlab.wikia.com/wiki/FAQ#How_do_I_create_a_circle.3F for several options on drawing circles.
Or you could create three identically sized vectors: the first being the points along the x-axis:
x = (-r:d:r)';
where r is the radius of the circle and d is the step size. The next vector is just the upper half of the circle according to the equation of the circle:
cu = sqrt(r^2 - x.^2);
And the third vector is just the lower half of the circle:
cl = -cu;
You can then apply "shifts" to the data to get the circle centred at coordinate (h,k), and then just plot both halves of the circle.
