title: Adding null values to a pandas dataframe
body: <p>I have a pandas dataframe that is used to create a JSON which in turn is used to display a highcharts chart.</p>
  
  <p>Pandas dataframe:</p>
  
  <pre><code>Date        colA    colB
  12-Sep-14   20      40
  13-Sep-14   50      10
  14-Sep-14   12      -20
  15-Sep-14   74      43
  </code></pre>
  
  <p>Is there a way to change some of the colA and colB values to null. The reason for this is that I ultimately need a JSON that looks something like this:</p>
  
  <pre><code>[
      [12-Sep-14, 20, 40],
      [13-Sep-14, null, null],
      [14-Sep-14, 12, -20],
      [15-Sep-14, 74, 43]
  ]
  </code></pre>
  
  <p>The reason for this is that I require a highcharts chart where certain plot points are blank. To do this, you specify the date followed by null.</p>
  
  <p>So I need to somehow update certain values in the pandas dataframe so that once I convert it to a JSON using .to_json() then the json will contain the specified null values as per the example above.</p>
  
  <p>Thanks for any suggestions.</p>