title: Donut chart in highcharts appears messed up on export
body: <p>We are using highcharts as our charting library. It's great and exporting works well in all scenarios except one. We have a donut chart with two levels. When it is rendered in the browser it shows up fine:</p>
  
  <p><img src="https://i.stack.imgur.com/4QS9J.png" alt="in browser"></p>
  
  <p>If you now export this chart using the default highcharts service it shows a bit like this:
  <img src="https://i.stack.imgur.com/fD2po.jpg" alt="exported image"></p>
  
  <p>Anyone know why this is happening and if there is any way we can fix this?</p>