Hide series, showing only one in the tooltip in Highcharts

I have several series in a Highcharts graphic but I only want to show the values of one of them in the tooltip. How would it be done?

Now with this code I show all the values of all the series:

tooltip: {
    formatter: function() {
        var s = '<b>'+ this.x +'</b>';
        var chart = this.points[0].series.chart; 
        var categories = chart.xAxis[0].categories;
        var index = 0;
        while(this.x !== categories[index]){index++;}          
        $.each(chart.series, function(i, series) { 
            s += '<br/>'+ series.name +': ' +series.data[index].y ;    
        });           
        return s;
    },
    shared: true
}