y

This property specifies the offset that is applied to the label text, along y.

axisY: {
        plotLines: [
                {
                    start: 6,
                    drawWidth: 2,
                    drawColor: 'red',
                   label: {
                        text: "High Line",
                        x:100,
                        y:-50,
                        style: {
                            color: "red"                          
                        }
                    }
                }
            ]
    },

This setting is demonstrated in the following example.