align

This property determines the vertical alignment of the text.

axisX: {
        plotLines: [
                {
                    start: 0.5,
                    drawWidth: 2,
                    drawColor: 'red',                    
                    label: {
                        text: "start value",
                        align:"center",
                        style: {
                            color: "red"
                        }
                    }
                }
            ]
    },

This setting is demonstrated in the following example.