jfreechart柱状图顶部显示数值？

我的程序是：

<%@ page contentType="text/html;charset=GBK"%>
<%@ page import="java.awt.BasicStroke,
                    java.awt.Color,
                    java.awt.Dimension,
                    javax.swing.JPanel,
                    org.jfree.chart.ChartFactory,
                    org.jfree.chart.ChartPanel,
                    org.jfree.chart.JFreeChart,
                    org.jfree.chart.labels.StandardCategoryItemLabelGenerator,
                    org.jfree.chart.axis.NumberAxis,
                    org.jfree.chart.plot.CategoryPlot,
                    org.jfree.chart.plot.PlotOrientation,
                    org.jfree.chart.servlet.ServletUtilities,
                    org.jfree.chart.renderer.category.BarRenderer3D,
                    org.jfree.chart.renderer.category.BarRenderer,
                    org.jfree.data.category.CategoryDataset,
                    org.jfree.data.category.DefaultCategoryDataset,
                    org.jfree.ui.ApplicationFrame,
                    org.jfree.ui.RefineryUtilities
 "%>

<%
String series1 = "额济纳旗";
String series2 = "阿拉善右旗";
String series3 = "阿拉善左旗";
String series4 = "乌拉特后旗";
// column keys...
String category1 = "";

// create the dataset...
DefaultCategoryDataset dataset = new DefaultCategoryDataset();
dataset.addValue(1.0, series1, category1);

dataset.addValue(5.0, series2, category1);

dataset.addValue(4.3, series3, category1);

dataset.addValue(3.0, series4, category1);

 // create the chart...
       JFreeChart chart =ChartFactory.createBarChart(
"阿拉善区各旗县人口密度2005年比较图", // chart title
"时间", // domain axis label
"人口密度值", // range axis label
dataset, // data
PlotOrientation.VERTICAL, // orientation
true, // include legend
true, // tooltips?
false // URLs?
);
// NOW DO SOME OPTIONAL CUSTOMISATION OF THE CHART...
// set the background color for the chart...
CategoryPlot plot = chart.getCategoryPlot();
       // customise the range axis...
       NumberAxis rangeAxis = (NumberAxis) plot.getRangeAxis();
       rangeAxis.setStandardTickUnits(NumberAxis.createIntegerTickUnits());
       rangeAxis.setAutoRangeIncludesZero(true);
       rangeAxis.setUpperMargin(0.20);
       rangeAxis.setLabelAngle(Math.PI / 90.0);   
      // BarRenderer3D renderer = (BarRenderer3D) plot.getRenderer();
       
       BarRenderer3D   renderer   =   new   BarRenderer3D();  
      // 设置每个地区所包含的平行柱的之间距离  
      renderer.setItemMargin(0.5);  
      // 显示每个柱的数值，并修改该数值的字体属性  
      //renderer.setBaseLabelItemGenerator(new   StandardCategoryItemLabelGenerator()); 
               
              renderer.setItemLabelsVisible(true); 
              plot.setRenderer(renderer);

 
String filename = ServletUtilities.saveChartAsPNG(chart, 400, 300, null, session);
String graphURL = request.getContextPath() + "/servlet/DisplayChart?filename=" + filename;
%>
<img src="<%= graphURL %>" width=400 height=300 border=0 usemap="#<%= filename %>">

希望大家能给我建议，解决一下，谢了！