title: 'matplotlib' has no attribute 'cm' when deploying an app
body: <p>I'm trying to deploy an app developed using Dash which use some matplotlib functions. When I run locally the application, it runs smoothly and everything is okay, however, when I deploy it using the same code (I'm deploying using heroku) the matplotlib version that is installed (which is the current 2.2.3) no longer find the attrbute 'cm' to create a colormap.</p>
  
  <pre><code>import matplotlib
  cmap = matplotlib.cm.get_cmap('Reds')
  </code></pre>
  
  <p><strong>Actual Outcome</strong></p>
  
  <p><a href="https://i.stack.imgur.com/T3awB.png" rel="noreferrer">Logs of the heroku app</a></p>
  
  <p>Matplotlib version</p>
  
  <p>I'm using Windows 10, with matplotlib 2.2.3 locally and in the deploy and, locally, my Python version is 3.6.6.</p>
  
  <p>Thank you!</p>
  
  <p>Kind regards,
  Renan</p>