highcharts tooltip does not working

    xAxis: { 
            categories: ["","","",""], 
            labels: {
            y: 20
            }
    },
    yAxis: {

            allowDecimals: false,
            min : 0,
            labels: {
            formatter: function () {
             return this.value/1000000;
                }  
    },
    tooltip: {
             formatter: function() {
                         return Highcharts.numberFormat(this.y,0);
             }
    },

if xAxis categories like ["","","",""] then tooltip is working, But if categories : ["1","2","3","4"] like this then not working please help,,,,,!