title: Draw / Create Scatterplots of datasets with NaN
body: <p>I want to draw a scatter plot using pylab, however, some of my data are <code>NaN</code>, like this:</p>
  
  <pre><code>a = [1, 2, 3]
  b = [1, 2, None]
  </code></pre>
  
  <p><code>pylab.scatter(a,b)</code> doesn't work. </p>
  
  <p>Is there some way that I could draw the points of real value while not displaying these <code>NaN</code> value?</p>