title: missing value in highcharts line graph results in no line, just points
body: <p>please take a look at this:</p>
  
  <p><a href="http://jsfiddle.net/2rNzr/" rel="noreferrer">http://jsfiddle.net/2rNzr/</a></p>
  
  <pre><code>var chart = new Highcharts.Chart({
  
      chart: {
          renderTo: 'container'
      },
  
      xAxis: {
          categories: ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec']
      },
  
      series: [{
          data: [29.9, '', 106.4, 129.2, 144.0, 176.0, 135.6, 148.5, 216.4, 194.1, 95.6, 54.4]
      }]
  
  });
  </code></pre>
  
  <p>you'll notice that the data has a blank value in it (the second value), which causes the line graph to display incorrectly.</p>
  
  <p>Is this a bug?</p>
  
  <p>What is the correct way of specifying a missing value so there will be a gap in the line graph? i.e. I would NOT want the line graph to simply draw a line between points 1 and 3.</p>
  
  <p>Many thanks for producing an excellent product!</p>
  
  <p>Tony.</p>