title: Highcharts Add multiple hidden value to shared tooltive
body: <p>it's my first qestion in stack !
  I try to add multiple hidden value in tooltip</p>
  
  <p>my data example :</p>
  
  <pre><code>var sample1= [{y:3.1,ext1:'14.5',ext2:'14.5',color:'#0ef43c'},{y:2.9,ext1:'14.2',ext2:'14.5',color:'#30ff21'},{y:2.9,ext1:'14.2',ext2:'14.5',color:'#42ff2c'},etc...
  var sample2= [{y:3.1,ext1:'14.5',ext2:'14.5',color:'#0ef43c'},{y:2.9,ext1:'14.2',ext2:'14.5',color:'#30ff21'},{y:2.9,ext1:'14.2',ext2:'14.5',color:'#42ff2c'},etc...
  </code></pre>
  
  <p>and my tooltip</p>
  
  <pre><code> tooltip: {
              shared: true,
                   formatter: function() { 
                      var txt = Highcharts.dateFormat("&lt;b&gt;%Hh&lt;/b&gt;", this.x);
                      txt += "&lt;br /&gt;&lt;b&gt;Sample 1: &lt;/b&gt;"+ this.points[0].point.y + " m - " + this.points[0].point.ext1 + "s" + this.points[0].point.ext2+ "s";
                      txt += "&lt;br /&gt;&lt;b&gt;Sample 2: &lt;/b&gt;"+ this.points[1].point.y + " m - " + this.points[1].point.ext1+ "s" + this.points[1].point.ext2+ "s";
                      txt += "&lt;br /&gt;Sample 3: :" + this.points[2].point.y + " m";
  
                      return txt;
  
          }
          }, 
  </code></pre>
  
  <p>But there is problem with hidden data , so is there a way for use a code as
  sample1.data.point.ext1
   instead of this.points[0].point.ext1 ?</p>
  
  <p>Thanks</p>