An optional setting on a PivotGroup that defines buckets for the values in the source data column rather than breaking out each individual value. Only one PivotGroup with a group rule may be added for each column in the source data, though on any given column you may add both a PivotGroup that has a rule and a PivotGroup that does not.
{
"dateTimeRule": {},
"histogramRule": {},
"manualRule": {}
}