Set Additional Data to highcharts series

Yes, if you set up the series object like the following, where each data point is a hash, then you can pass extra values:

new Highcharts.Chart( {
    ...,
    series: [ {
        name: 'Foo',
        data: [
            {
                y : 3,
                myData : 'firstPoint'
            },
            {
                y : 7,
                myData : 'secondPoint'
            },
            {
                y : 1,
                myData : 'thirdPoint'
            }
        ]
    } ]
} );

In your tooltip you can access it via the "point" attribute of the object passed in:

tooltip: {
    formatter: function() {
        return 'Extra data: <b>' + this.point.myData + '</b>';
    }
}

Full example here: https://jsfiddle.net/burwelldesigns/jeoL5y7s/

Additionally, with this solution, you can even put multiple data as much as you want :

tooltip: {
    formatter: function () {
        return 'Extra data: <b>' + this.point.myData + '</b><br> Another Data: <b>' + this.point.myOtherData + '</b>';
    }
},

series: [{
    name: 'Foo',
    data: [{
        y: 3,
        myData: 'firstPoint',
        myOtherData: 'Other first data'
    }, {
        y: 7,
        myData: 'secondPoint',
        myOtherData: 'Other second data'
    }, {
        y: 1,
        myData: 'thirdPoint',
        myOtherData: 'Other third data'
    }]
}]

Thank you Nick.

For time series data, especially with enough data points to activate the turbo threshold, the proposed solutions above will not work. In the case of the turbo threshold, this is because Highcarts expects the data points to be an array like:

series: [{
    name: 'Numbers over the course of time',
    data: [
      [1515059819853, 1],
      [1515059838069, 2],
      [1515059838080, 3],
      // you get the idea
    ]
  }]

In order not to lose the benefits of the turbo threshold (which is important when dealing with lots of data points), I store the data outside of the chart and look up the data point in the tooltip formatter function. Here's an example:

const chartData = [
  { timestamp: 1515059819853, value: 1, somethingElse: 'foo'},
  { timestamp: 1515059838069, value: 2, somethingElse: 'bar'},
  { timestamp: 1515059838080, value: 3, somethingElse: 'baz'},
  // you get the idea
]

const Chart = Highcharts.stockChart(myChart, {
  // ...options
  tooltip: {
    formatter () {
      // this.point.x is the timestamp in my original chartData array
      const pointData = chartData.find(row => row.timestamp === this.point.x)
      console.log(pointData.somethingElse)
    }
  },
  series: [{
      name: 'Numbers over the course of time',
      // restructure the data as an array as Highcharts expects it
      // array index 0 is the x value, index 1 is the y value in the chart
      data: chartData.map(row => [row.timestamp, row.value])
    }]
})

This approach will work for all chart types.