title: Pylab contour plot using Mollweide projection create artefacts
body: <p>The purpose of the script is to do a contour plot of some data in a Mollweide projection with pylab. There is a strange behaviour of <code>contourf</code>. Here is the code</p>
  <pre><code>import numpy as np
  import pylab as plt
  
  ra = np.linspace(-np.pi, np.pi, 40)
  dec= np.linspace(-np.pi/2, np.pi/2, 20)
  X,Y = np.meshgrid(ra,dec)
  Z = np.sin(X) * np.cos(X) * np.sin(Y) * np.cos(Y)
  
  plt.figure()
  ax = plt.subplot(111, projection = 'mollweide')
  ax.contourf(X,Y,Z,100)
  ax.contour(X,Y,Z,10,colors='k')
  
  plt.show()
  </code></pre>
  <p>Some of the points seem to be wrongly drawn (See Figure below).</p>
  <p><img src="https://i.stack.imgur.com/Vpqsm.png" alt="Mollweide" /></p>
  <p>The same code used without projection outputs a perfectly normal map:
  <img src="https://i.stack.imgur.com/yBDml.png" alt="enter image description here" /></p>
  <p>You may have noticed that the contour lines work perfectly in both images. The artefacts seem to be always at the same place.</p>
  <p>Is there a way to correct this artefacts generation ?</p>