[Solved]-Highcharts - How to get a value of a stack in a series?-Highcharts

the value of the stack option can be found in series.options.stack.

for your code it would look like this (jsfiddle):

tooltip: {
    formatter: function () {
        return '<b>' + this.x + '</b><br/>' +
            this.series.name + ': ' + this.y + '<br/>' +
            'total: ' + this.point.stacktotal + ' stack: ' + this.series.options.stack;
    }
}