title: matplotlib unexpected results polar plot
body: <p>I am trying to plot simple function r = 3*sin(2*theta) using matplotlib:</p>
  
  <pre class="lang-py prettyprint-override"><code>import numpy as np
  import matplotlib.pyplot as plt
  theta = np.arange(0,2*np.pi,0.01)
  r = 3.0*np.sin(2.0*theta)
  ax = plt.subplot(111, projection='polar')
  ax.plot(theta, r)
  plt.show()
  </code></pre>
  
  <p>This is the result I get (it is not correct):</p>
  
  <p><img src="https://i.stack.imgur.com/6Kp6D.png" alt="matplolib"></p>
  
  <p>This is what I expect to see (wolfram alpha): <img src="https://i.stack.imgur.com/vhqDo.png" alt="wolfram alpha"></p>
  
  <p>Am I missing something?
  Thanks!</p>