format

A format string / function, allowing to override the text displayed for the axis ticks.

Format string example:

axisY: {
    axisTickText: {
        // format the text as a float with one digit precision
        format: "{point.y:n1}"
    }
}

Format function example:

axisY: {
    axisTickText: {
        // return a new text for the ticks of the Y-axis
        format: function(text, axis, chart) {
            return text + "%%";
        }
    }
},

This setting is demonstrated in the following example.