title: HighChart IE9 z-index issue
body: <p>I have a HTML select above the highchart control. The from and to fields are on top of the dropdown (see image). I've tried setting the z-index of the select and the chart container. Nothing seems to work. This only happens in IE. Any advice?</p>
  
  <p><img src="https://i.stack.imgur.com/g8tGZ.jpg" alt="Example"></p>