class MyDataProvider implements IDataProvider {
  public Object getDataValue(int columnIndex, int rowIndex) {
    // figure out what group you're in based on the row index, then return the appropriate data or summary data
  }

  public int getRowCount() {
    return underlyingDataRowCount
      + numGroups  // for each group summary row
      + 1;  // for the overall total row
  }