title: draw an ellipse or contour in matlab
body: <p>I have two vectors a = [1 1]' and b = [1 -1]' which are linearly independent.
  I want to draw a shape like an ellipse or a contour around these points, so I can see the area which is spanned by these two vectors.</p>
  
  <p>The picture below shows what I want to get. One of the blue vectors belongs to a and one of the red to b (I drew the mirrored vectors also for demonstration purpose). The green circle is what I want to draw.
  <img src="https://i.stack.imgur.com/pRkz7.png" alt="enter image description here"></p>
  
  <p>How can I do that?</p>