title: pyplot.contourf() returns error when specified levels argument
body: <p><strong>EDIT: The issue is most likely about the version. The <code>levels</code> argument takes an integer argument in version 3.0.0, while this issue has occurred while using version 2.2.2.</strong></p>
  
  <p><strong>UPDATE: The issue didn't occur after installing the version >=3.0.0.</strong></p>
  
  <p>I'm trying to make a contour plot in Python using the <code>matplotlib.pyplot.contourf()</code> function, and it works perfectly like this:</p>
  
  <pre><code>plt.contourf(x, y, z)
  </code></pre>
  
  <p>but when I try to specify an integer for the levels argument, like this:</p>
  
  <pre><code>plt.contourf(x, y, z, levels=100)
  </code></pre>
  
  <p>it always returns the error: <code>TypeError: len() of unsized object</code></p>
  
  <p>In the documentation, it says that the argument <code>levels</code> can be either <code>int</code> or <code>array_like</code> so I don't know why it would even call the <code>len()</code> function</p>
  
  <p>Any ideas why this happens and any suggestion on how to fix it?</p>