More rows in column header

Question

I would like to get a table header like this:

But I'm able to get only column header without the third row (y1, y2, y3). How can I do that?

I have something like this so far:

protected ILayer createColumnHeaderLayer(IDataProvider dataProvider, 
                                         ILayer bodyLayer, 
                                         IUniqueIndexLayer dataLayer, 
                                         SelectionLayer selectionLayer) {

    IDataProvider columnHeaderDataProvider = new TeppichDiagramHeaderDataProvider(dataProvider);
    DataLayer columnBaseLayer = new DataLayer(
            columnHeaderDataProvider, 
            DataLayer.DEFAULT_COLUMN_WIDTH, 
            30
    );
    ColumnHeaderLayer columnHeaderLayer = new ColumnHeaderLayer(columnBaseLayer, bodyLayer, selectionLayer, false);

    ColumnGroupModel columnGroupModel = new ColumnGroupModel();
    ColumnGroupHeaderLayer columnGroupHeaderLayer = new ColumnGroupHeaderLayer(
            columnHeaderLayer, 
            selectionLayer, 
            columnGroupModel,
            false
    );
    columnGroupHeaderLayer.addColumnsIndexesToGroup(
            "Group", 
            3, 4, 5
    );

    return columnGroupHeaderLayer;
}

where TeppichDiagramHeaderDataProvider is a dataprovider for the column header. Where it returns 1 from its getRowCount() method.