use fill
for that, for example:
r=5000; % some radius
color=[1 0 0]; % red color
t=linspace(0,2*pi);
fill(E+r*cos(t),N+r*sin(t),color); % this creates a filled circle centered at (E,N)
note that though this creates a filled circle, when you actually plot, the axes are set automatically, so it may looks like an ellipse. In order to make it look right in any zoom, some more work needs to be done to the fill
line. But that's the jist. (or use axis square
)