title: How to include data from point in Highcharts xAxis label?
body: <p>I am trying to include some extra data from my points in the xAxis  label of a Highcharts line chart.  </p>
  
  <p>I'm creating my points like this.  xAxis is 'datetime' type, and I'm using custom tickPositions (this part works fine).</p>
  
  <pre><code>for (row of results) {
      var point = {x:Date.parse(row.time), y:row.value, magicNumber:row.ID};
      data_series.data.push(point);
      chartConfig.xAxis.tickPositions.push(Date.parse(row.time));
  }
  </code></pre>
  
  <p>In the tooltip I'm able to do the following (this works):</p>
  
  <pre><code>tooltip: {          
      formatter: function() {
          return 'ID: ' + this.point.magicNumber + '  Value:' + this.point.y.toFixed(3);
      },
  },
  </code></pre>
  
  <p>How do I do the equivalent in the xAxis formatter?  Not clear from <a href="https://api.highcharts.com/highcharts/xAxis.labels.formatter" rel="nofollow noreferrer">the docs</a> if this is possible.</p>
  
  <pre><code> xAxis: {
          type: 'datetime',
          labels: {
              rotation: 90,    
              formatter: function () {
                  var ID = **What goes here to obtain magicNumber?** 
  
                  var datetime = new Date(this.value);
                  return ID.toString() + ' ' + datetime.toISOString();
              }
          },
  
          tickPositions: []
  }
  </code></pre>