You can control the format of the tooltip using the formatter function in the tooltip block. For example:

 tooltip: {
        formatter: function () {
            var s = '<b>' + this.x + '</b>';

            $.each(this.points, function (i, point) {
                s += '<br/>' + + ': ' + point.y;

            return s;
        shared: true

This formats the tooltip using a header of the x value, and lists series name and y for each series. shared: true means the tooltip is shared between both series, appearing the same no matter which series you hover over.

There is plenty of documentation on the tooltips along with some examples here:

and also here:

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