# Welcome to the Highcharts Gantt JS (gantt) Options Reference

These pages outline the chart configuration options, and the methods and properties of Highcharts objects.

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# rangeSelector.buttons.dataGrouping.approximation

The method of approximation inside a group. When for example 30 days are grouped into one month, this determines what value should represent the group. Possible values are "average", "averages", "open", "high", "low", "close" and "sum". For OHLC and candlestick series the approximation is "ohlc" by default, which finds the open, high, low and close values within all the grouped data. For ranges, the approximation is "range", which finds the low and high values. For multi-dimensional data, like ranges and OHLC, "averages" will compute the average for each dimension.

Custom aggregate methods can be added by assigning a callback function as the approximation. This function takes a numeric array as the argument and should return a single numeric value or `null`. Note that the numeric array will never contain null values, only true numbers. Instead, if null values are present in the raw data, the numeric array will have an `.hasNulls` property set to `true`. For single-value data sets the data is available in the first argument of the callback function. For OHLC data sets, all the open values are in the first argument, all high values in the second etc.

Since v4.2.7, grouping meta data is available in the approximation callback from `this.dataGroupInfo`. It can be used to extract information from the raw data.

Defaults to `average` for line-type series, `sum` for columns, `range` for range series, `hlc` for HLC, and `ohlc` for OHLC and candlestick.