title: How to edit the tooltip text in a highcharts boxplot
body: <p>I want to change the text of the popup on a box plot.</p>
  
  <p>From the <a href="http://api.highcharts.com/highstock#tooltip.formatter" rel="nofollow noreferrer">api</a> and the example linked there, I assumed it would be a case of adding a formatter function to the series.  So I went to the <a href="http://www.highcharts.com/demo/box-plot" rel="nofollow noreferrer">demo</a> and clicked 'Edit in jsFiddle'. I then changed:</p>
  
  <pre><code>tooltip: {
      headerFormat: '&lt;em&gt;Experiment No {point.key}&lt;/em&gt;&lt;br/&gt;'
  }
  </code></pre>
  
  <p>to</p>
  
  <pre><code>tooltip: {
      headerFormat: '&lt;em&gt;Experiment No {point.key}&lt;/em&gt;&lt;br/&gt;',
      formatter: function() { return 'some random string'; }
  }
  </code></pre>
  
  <p>I expected the tooltip to change to 'some random string' (as happens in the demo linked from the tooltip api reference), but it was unchanged.  Any hints?</p>