I've been working on a Highstock.

Why when I do "this.series.name" (in tooltip: {formatter:...}) I got an error "Cannot read property 'name' of undefined" ?
Whereas "this.x" work.

Example of the problem:

```
tooltip: {
    formatter: function () {
      // work:
         //var s = '<b>' + this.x + '</b>';
      // does not work:
         var s = '<b>' + this.series.name + '</b>';

        $.each(this.points, function () {
            s += '<br/>1 USD = ' + this.y + ' EUR';
        });

        return s;
    }
},

```

<http://jsfiddle.net/2sssgfLz/>

Doc of the API:
<http://api.highcharts.com/highstock/tooltip.formatter>