[Solved]-Can I add an "average" line to Highcharts Chart?-Highcharts

Yes. You can add it as a plotline, like this:

yAxis: {
    // ...Options
    plotLines: [{
        color: 'red',
        value: '15', // Insert your average here
        width: '1',
        zIndex: 4 // To not get stuck below the regular plot lines or series
    }]
}

See this JSFiddle demonstration.