Question

How do i add a unit for my individual Y axis?
I have 4 axis, all with different units (C, hPa, %)

My tooltip is currently like this:

tooltip: {
    formatter: function() {
    return ''+
    Highcharts.dateFormat('%H:%M',this.x) +': '+ this.y;
    }
}

Answer 1

You can change the tooltip according to series.

Example

tooltip: {
    formatter: function() {
        if(this.series.name === 'Temperature') {
            return ''+ Highcharts.dateFormat('%H:%M',this.x) +': '+ this.y + 'C';
        } else {
            return ''+ Highcharts.dateFormat('%H:%M',this.x) +': '+ this.y + '%';
        }
    }
}

Demo