[Solved]-HighChart Tooltip addition text and value-Highcharts

Declare tooltip options with formatter function:

options: {
    ...,
    tooltip: {
            formatter: function () {
            return 'The value for <b>' + this.x +
                '</b> is <b>' + this.y + '</b>' 
              + '<br/>'  + 'Extra data: <b>' + this.point.extra + '</b>';
        },
    }
}

And define series data like below:

data:[
   {
      y: 49.9, 
      extra: 49.9
   },
   {
      y: 71.5,
      extra: 71.5
   },
   ...
],

See jsfiddle code here