Just increase the zIndex of the line series to a higher value, 1000 for example.

Demo: https://jsfiddle.net/BlackLabel/6qybs0Lt/

API reference: https://api.highcharts.com/highcharts/series.line.zIndex

{
    name: 'Average',
    type: 'line',
    data: [4, 2.67, 2.67, 3.33, 1.33, 2.67, 3.67, 3.00, 2.67, 2.33, 2.67, 3.00, 3.67, 4.00, 4.67, 4.00, 4.00, 3.00, 2.67, 2.00, 2.00, 2.00, 2.00, 1.67],
    zIndex: 1000
}