title: Highcharts display grid over plot with z-index
body: <p>I'm trying to display a graph with the xAxis and yAxis grid lines overlayed on top of the plot.</p>
  
  <p>I can set the zIndex of the plot, but there doesn't seem to be an option to set the zIndex of the grid.
  If it's not possible, I can lower the opacity of the plot so that the grid shows through from underneath, but I'd rather not do it that way.</p>