title: matplotlib colorbar smoothness
body: <p>I am trying to plot some data using contourf using this function:</p>
  
  <pre><code>cmap = mpl.cm.jet
  norm = mpl.colors.Normalize(vmin=min(zi), vmax=max(zi))
  ax = plt.contourf(xi, yi, zi, N=20, norm=norm, cmap=cmap)
  plt.colorbar(ax)
  </code></pre>
  
  <p>The colorbar I am getting is this one:</p>
  
  <p><img src="https://i.imgur.com/gLcqy.png" alt="alt text"></p>
  
  <p>However, I would like to have the colorbar more gradient such as this:</p>
  
  <p><img src="https://i.imgur.com/erjYt.png" alt="alt text"></p>
  
  <p>Is it possible to have it?  I tried adding N=50 to the contourf function but it did not help. </p>