formatter property

Callback JavaScript function to format the label. The value is given by this.value. Additional properties for this are axis, chart, isFirst, isLast and text which holds the value of the default formatter. Since v12.6.0, the callback also receives ctx as the first argument, so that arrow functions can access the same context as regular functions using this.

Defaults to a built in function returning a formatted string depending on whether the axis is category, datetime, numeric or other.

API Docs: https://api.highcharts.com/highcharts/chart.parallelAxes.labels.formatter