title: Add horizontal reference line to Google charts' LineChart
body: <p>I'm using <a href="https://developers.google.com/chart/interactive/docs/gallery/linechart" rel="nofollow">Google visualization's LineChart</a> to display some data (it works).</p>
  
  <p>The chart shows performance tests results and those results should <em>not</em> be over some value (eg. response time should not be more than 20ms). Si I'd like to draw that maximum (an horizontal line I guess) without having to add a new (dummy) series of data. </p>
  
  <p>Is that possible?</p>
  
  <p>Thanks a lot, </p>
  
  <p>Alban</p>