title: Plot a cone with matplotlib
body: <p>I'm new to plotting graphs in 3D and would like to plot a cone in 3D using matplotlib. It should start with something like this:</p>
  
  <pre><code>import matplotlib.pyplot as plt
  from mpl_toolkits.mplot3d import Axes3D
  fig = plt.figure()
  ax = fig.add_subplot(111, projection='3d')
  </code></pre>