These pages outline the chart configuration options, and the methods and properties of Highcharts objects.
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Normally, a group is indexed by the start of that group, so for example when 30 daily values are grouped into one month, that month's x value will be the 1st of the month. This apparently shifts the data to the left. When the smoothed option is true, this is compensated for. The data is shifted to the middle of the group, and min and max values are preserved. Internally, this is used in the Navigator series.