Fires when the minimum and maximum is set for the axis, either by calling the .setExtremes() method or by selecting an area in the chart. The this keyword refers to the axis object itself. One parameter, event, is passed to the function. This contains common event information based on jQuery or MooTools depending on which library is used as the base for Highcharts. The new user set minimum and maximum values can be found by event.min and event.max.