title: Projection on a plane using a 2*3 matrix
body: <p>I can easily draw the projection of a 3D set of points onto the plane with normal vector (1,1,1), by using the matrix</p>
  
  <pre><code>    (-sqrt(3)/2   sqrt(3)/2   0)
      (-1/2         -1/2        1).
  </code></pre>
  
  <p>I want to do the same thing, but for a projection onto an arbitrary plane with normal vector (a,b,c) instead of (1,1,1). How to find the matrix?</p>
  
  <p>Thanks in advance!</p>
  
  <p><b>EDIT: rephrasing of the question:</b></p>
  
  <p>When viewing from (1,1,1), the three unit vectors are projected to (-sqrt(3)/2, -1/3), (sqrt(3)/2, -1/2), (0,1). (Up to a scaling factor which doesn't matter.)</p>
  
  <p>I want to find the projection of the three unit vectors when viewed from (a,b,c) instead of (1,1,1).</p>