title: plotting a projection of a circle on a plane
body: <p>I'm trying to plot the 3D-plot in the first example here: <a href="http://en.wikipedia.org/wiki/Lagrange_multiplier#Examples" rel="nofollow">http://en.wikipedia.org/wiki/Lagrange_multiplier#Examples</a>. I've managed to plot the surface:</p>
  
  <pre><code>[x,y]=meshgrid(-3:0.2:3);
  z=x+y;
  mesh(x,y,z)
  </code></pre>
  
  <p>Any ideas on how to project the circle on the surface? </p>