title: Drawing circles of variable size and position on a 2d animation
body: <p>I am working with matplotlib in Python 3.3. I have an animated 2d and 3d window, on which I draw points. These points represent objects, but it is not certain that they are actually there. So I want to draw a circle around these points to display the uncertainty. This uncertainty is variating, so bottomline: I want to draw multiple (might be 0, might be 100) circkles in a 2D animation with a variable center and a variable radius.</p>
  
  <p>I tried scatter. Scatter looks very promising, until I resize the screen: the size of the scatter is in pixels and not related to the size of the axes. Any idea how I can do this?</p>
  
  <p>I have tried the following scatter code:</p>
  
  <pre><code>#In the init
  self.circles = self.ax.scatter([],[],[],c = 'b',alpha=0.6)
  #In the animate function
  self.circles.set_offsets([[10],[15]])    
  self.circles._sizes = [2000]
  </code></pre>
  
  <p>The centerpoint is correct, at x = 10 and y = 15. But the size is the size in pixels and not related to the axes. In this case I expected the circle to hit x = 10 and y = 20015, for example. But it does not. </p>
  
  <p>This becomes a problem when I resize the window.</p>