title: Issue in generating thumbnail for heatmap highcharts graph
body: <p>I have been generating thumbnail of few of my highcharts graphs. It was working fine but for few days, the thumbnail of one of the graphs (which is a heatmap chart) is not rendering properly. Instead, it prints "Chart input data error (Highcharts error #17) " error message in the box where thumbnail was generated earlier. I have looked about details of the error at <a href="http://www.highcharts.com/errors/17" rel="nofollow noreferrer">http://www.highcharts.com/errors/17</a> and I was able to identify that the issue is due to "chart type : heatmap" but my point is that earlier it was working fine. So, what can be the reason of the issue now?
  For reference, I have create a fiddle at <a href="http://jsfiddle.net/6c48x39v/14/" rel="nofollow noreferrer">http://jsfiddle.net/6c48x39v/14/</a>   <code>Heatmap thumbnail</code></p>