contour plot + lines

[Claudio Belotti]

Hi all,
I have 

[xx,yy]=meshgrid(x,y)
zz=f(xx,yy)

and I plot zz with 
contour(xx,yy,zz)

now I would like to plot some lines on this plot something like and/or some 
points

I tried (with hold on)
plot3(0,0,500,"r+");
and 
plot(0,0,"r+")

but I got only "Notice: cannot contour non grid data!"

how can I do?

thanks

Claudio



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