clf;
x=linspace(-3,3,100);
y=x;
[xx,yy]=meshgrid(x,y);
z_1=(xx.^2 - yy.^2) ./2;
contour(xx,yy,z_1,-3:0.5:3); 
hold on; 

z_2=xx.*yy; 
contour(xx,yy,z_2,-3:0.5:3); 
hold on;

grid on;
axis equal; 
%set(gca,'PlotBoxAspectRatio',[1 1]);