title: plot a circle with pyplot
body: <p>surprisingly I didn't find a straight-forward description on how to draw a circle with matplotlib.pyplot (please no pylab) taking as input center (x,y) and radius r. I tried some variants of this:</p>
  
  <pre><code>import matplotlib.pyplot as plt
  circle=plt.Circle((0,0),2)
  # here must be something like circle.plot() or not?
  plt.show()
  </code></pre>
  
  <p>... but still didn't get it working. </p>