style

This property specifies the style that can be applied to the text. Style is a set of standard css properties, applicable to text.

axisY: {
        plotLines: [
                {
                    start: 6,
                    drawWidth: 2,
                    drawColor: 'red',
                    label: {
                        text: "High Line",
                        textAngle:-45,
                        style: {
                            color: "red", 
                            fontSize:23,
                            textDecoration:"underline"
                        }
                    }
                }
            ]
    },

This setting is demonstrated in the following example.