title: Show Gridline on Y axis for 0 point only
body: <p>I have a HighCharts bar graph that contains both negative and positive points.  </p>
  
  <p>I have <code>gridlines</code> turned off but ideally I would like to have the grid line showing for the <code>yAxis 0</code> point.</p>
  
  <p>I have tried doing another series along this point however this does not extend the full width of the graph and only goes from the first point to the last point. </p>
  
  <p>My graph does not have <code>endOnTicks</code>.</p>