highcharts - show label for first and last data points in a series

gistfile1.js

plotOptions: {
            line : {
              dataLabels : {
                enabled : true,
                formatter: function() {
                  var first = this.series.data[0],
                      last  = this.series.data[this.series.data.length - 1];
                  if ((this.point.category === first.category && this.point.y === first.y) ||
                      (this.point.category === last.category  && this.point.y === last.y)) {
                    return this.point.y;
                  }
                  return "";
                }
              },
            },
}