nullFormatter property

Callback JavaScript function that defines formatting for points with the value of null. Works analogously to formatter. nullFormatter can be applied only to series which support displaying null points. heatmap and tilemap supports nullFormatter by default while the following series requires #series.nullInteraction set to true: line, spline, area, area-spline, column, bar, and timeline. Does not work with series that don't display null points, like pie.

API Docs: https://api.highcharts.com/highcharts/series.treegraph.dataLabels.nullFormatter