Accepted answer

While I am uncertain as to why your solution doesn't work, I can propose an alternative solution.

The tooltip formatter function has access to a number of different parameters. Instead of this.x, you could use

For example:

formatter: function() {
    // If you want to see what is available in the formatter, you can
    // examine the `this` variable.
    //     console.log(this);

    return '<b>'+ Highcharts.numberFormat(this.y, 0) +'</b><br/>'+
        'in year: '+;

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