[kc121]

这样解决更好。例子：
[x,y] = meshgrid(-2:0.1:2);
z = y.*exp(-x.^2-y.^2);
z1=z;
z2=z;
z1(z1<0)=0;
z2(z2>0)=0;

contour(x,y,z1,50,'-');
hold on
contour(x,y,z2,50,':');