x = linspace(-3,3,100);
y = x;
[X,Y]=meshgrid(x,y);
k = 1;
for r=[0 0.5 1 2]
    figure(k);
    mesh(X,Y,sqrt(X.^2+Y.^2));
    hold on
    mesh(X,Y,r*X+1);
    hold off
    k = k + 1;
end