label

Specifies the text that is applied to the custom plotline.

axisX: {
        plotLines: [
                {
                    start: 6,
                    drawWidth: 2,
                    drawColor: 'red',
                    label: {
                        text: "High Value",
                        style: {
                            color: "red"
                        }
                    }
                }
            ]
    },

This setting is demonstrated in the following example.