[Solved]-With HighCharts, how can I have the same tooltip formatter as my y axis-Highcharts

you must set the tooltip formatter api like that :

tooltip:{
  ...
  formatter: function(){
    var text = '';
    if(this.series.index == 0) {
        text = this.series.name + ' : ' + this.y/1000 + 'k';
    } else {
        text = '<b>' + this.series.name + '</b> : ' + this.y ;
    }
    return text;
  }
}

fiddle

edit for multiple yaxis