title: Append to default highcharts tooltip
body: <p>I have a line graph with the y-axis set to category and x-axis to values. On hovering over a point , i get the default tooltip information like in 
  <a href="http://jsfiddle.net/gh/get/library/pure/highcharts/highcharts/tree/master/samples/highcharts/plotoptions/series-point-events-mouseover/" rel="nofollow noreferrer">http://jsfiddle.net/gh/get/library/pure/highcharts/highcharts/tree/master/samples/highcharts/plotoptions/series-point-events-mouseover/</a> when </p>
  
  <pre><code> tooltip: {
          enabled: true
      }
  </code></pre>
  
  <p>My y-axis is set to some label like "Seconds" or "FPS". I want to be able to just add the y-axis label text to the default tooltip without changing any formatting options in the default tootlip, so for example if you hover over the first point it should display <strong>29.9 Seconds</strong> along with the other data keeping other contents unaltered.</p>
  
  <p>I know you can use a formatter and override it but I would like to retain the original tooltip display which shows up when you dont override the formatter.</p>