http://www.extjs.com/examples-dev/ex...egationrowgrid this example work fine..

but if you would like group one cell.. broken..

code example
Code:
package com.extjs.gxt.samples.client.examples.grid;

import java.util.ArrayList;
import java.util.List;

import com.extjs.gxt.samples.resources.client.TestData;
import com.extjs.gxt.samples.resources.client.model.Stock;
import com.extjs.gxt.ui.client.Style.HorizontalAlignment;
import com.extjs.gxt.ui.client.store.ListStore;
import com.extjs.gxt.ui.client.widget.ContentPanel;
import com.extjs.gxt.ui.client.widget.LayoutContainer;
import com.extjs.gxt.ui.client.widget.grid.AggregationRenderer;
import com.extjs.gxt.ui.client.widget.grid.AggregationRowConfig;
import com.extjs.gxt.ui.client.widget.grid.ColumnConfig;
import com.extjs.gxt.ui.client.widget.grid.ColumnData;
import com.extjs.gxt.ui.client.widget.grid.ColumnModel;
import com.extjs.gxt.ui.client.widget.grid.Grid;
import com.extjs.gxt.ui.client.widget.grid.GridCellRenderer;
import com.extjs.gxt.ui.client.widget.grid.HeaderGroupConfig;
import com.extjs.gxt.ui.client.widget.grid.SummaryType;
import com.extjs.gxt.ui.client.widget.layout.FitLayout;
import com.extjs.gxt.ui.client.widget.layout.FlowLayout;
import com.extjs.gxt.ui.client.widget.table.NumberCellRenderer;
import com.google.gwt.i18n.client.DateTimeFormat;
import com.google.gwt.i18n.client.NumberFormat;
import com.google.gwt.user.client.Element;

public class AggregationGridExample extends LayoutContainer {

	@Override
	protected void onRender(Element parent, int index) {
		super.onRender(parent, index);

		setLayout(new FlowLayout(15));

		final NumberFormat currency = NumberFormat.getCurrencyFormat();
		final NumberFormat number = NumberFormat.getFormat("0.00");
		final NumberCellRenderer<Grid<Stock>> numberRenderer = new NumberCellRenderer<Grid<Stock>>(currency);

		GridCellRenderer<Stock> change = new GridCellRenderer<Stock>() {
			public String render(Stock model, String property, ColumnData config, int rowIndex, int colIndex, ListStore<Stock> store,
					Grid<Stock> grid) {
				double val = (Double) model.get(property);
				String style = val < 0 ? "red" : "green";
				return "<span style='color:" + style + "'>" + number.format(val) + "</span>";
			}
		};

		GridCellRenderer<Stock> gridNumber = new GridCellRenderer<Stock>() {
			public String render(Stock model, String property, ColumnData config, int rowIndex, int colIndex, ListStore<Stock> store,
					Grid<Stock> grid) {
				return numberRenderer.render(null, property, model.get(property));
			}
		};

		List<ColumnConfig> configs = new ArrayList<ColumnConfig>();

		ColumnConfig column = new ColumnConfig();
		column.setId("name");
		column.setHeader("Company");
		column.setWidth(200);
		column.setToolTip("sfsdf");
		configs.add(column);

		column = new ColumnConfig();
		column.setId("symbol");
		column.setHeader("Symbol");
		column.setWidth(100);
		configs.add(column);

		column = new ColumnConfig();
		column.setId("last");
		column.setHeader("Last");
		column.setAlignment(HorizontalAlignment.RIGHT);
		column.setWidth(100);
		column.setRenderer(gridNumber);
		configs.add(column);

		column = new ColumnConfig("change", "Change", 100);
		column.setAlignment(HorizontalAlignment.RIGHT);
		column.setRenderer(change);
		configs.add(column);

		column = new ColumnConfig("date", "Last Updated", 100);
		column.setAlignment(HorizontalAlignment.RIGHT);
		column.setDateTimeFormat(DateTimeFormat.getShortDateFormat());
		configs.add(column);

		final ListStore<Stock> store = new ListStore<Stock>();
		store.add(TestData.getStocks());

		ColumnModel cm = new ColumnModel(configs);

		cm.addHeaderGroup(0, 1, new HeaderGroupConfig("Stock Information", 1, 1));

		AggregationRowConfig<Stock> totals = new AggregationRowConfig<Stock>();
		totals.setHtml("name", "Average");

		totals.setSummaryType("last", SummaryType.AVG);
		totals.setRenderer("last", new AggregationRenderer<Stock>() {
			public Object render(Number value, int colIndex, Grid<Stock> grid, ListStore<Stock> store) {
				return NumberFormat.getCurrencyFormat().format(value.doubleValue());
			}
		});

		totals.setSummaryType("change", SummaryType.AVG);
		totals.setRenderer("change", new AggregationRenderer<Stock>() {
			public Object render(Number value, int colIndex, Grid<Stock> grid, ListStore<Stock> store) {
				return number.format(value.doubleValue());
			}
		});
		cm.addAggregationRow(totals);

		totals = new AggregationRowConfig<Stock>();
		totals.setHtml("name", "Maximum");

		totals.setSummaryType("last", SummaryType.MAX);
		totals.setSummaryFormat("last", NumberFormat.getCurrencyFormat());

		totals.setSummaryType("change", SummaryType.MAX);
		totals.setRenderer("change", new AggregationRenderer<Stock>() {
			public Object render(Number value, int colIndex, Grid<Stock> grid, ListStore<Stock> store) {
				return number.format(value.doubleValue());
			}
		});
		cm.addAggregationRow(totals);

		ContentPanel cp = new ContentPanel();
		cp.setCollapsible(true);
		cp.setFrame(true);
		cp.setHeading("Aggregation Rows");
		cp.setButtonAlign(HorizontalAlignment.CENTER);
		cp.setLayout(new FitLayout());
		cp.setBounds(10, 10, 600, 350);

		Grid<Stock> grid = new Grid<Stock>(store, cm);
		grid.setBorders(false);
		grid.setAutoExpandColumn("name");
		grid.setBorders(true);
		cp.add(grid);

		add(cp);
	}

}
pay atention in this line

Code:
cm.addHeaderGroup(0, 1, new HeaderGroupConfig("Stock Information", 1, 1));