def groupByData = api.newMatrix()
             //Definition of result matrix (columns, rows).
             .withColumns('TextColumn1', 'TextColumn2', 'NumericColumn3', 'NumericColumn4')
             .withRows(entries)
             //Property `.withGroupBy` defines that the `TextColumn1` and `NumericColumn3` are used for group by (The order is important).                     
             .withGroupBy(['TextColumn1', 'NumericColumn3'])
             //Property `.withColumnAggregation` defines that the values for subtotal rows are based on the group by definition (`NumericColumn3`) and are to be calculated as SUM. This definition is optional. When not used, like e.g. for TextColumn1, there will be no aggregation for the `TextColumn1`.
             .withColumnAggregation('NumericColumn3', SUM)
             //Property `.calculateGroupByData` is an end statement for group definition. This statement is only used together with `.withColumnAggregation`.
             .calculateGroupByData()