jfreechart饼图有关问题

<%@ page contentType="text/html;charset=GBK"%><%@ page import="org.jfree.chart.*,                 org.jfree.chart.plot.PiePlot,              org.jfree.data.general.DefaultPieDataset,org.jfree.chart.servlet.ServletUtilities,                 java.awt.*"%><%//设置数据集DefaultPieDataset dataset = new DefaultPieDataset();dataset.setValue("初中高级程序员", 0.55);dataset.setValue("项目经理", 0.1);dataset.setValue("系统分析师", 0.1);dataset.setValue("软件架构师", 0.1);dataset.setValue("其他", 0.2);//通过工厂类生成JFreeChart对象JFreeChart chart = ChartFactory.createPieChart3D("IT行业职业分布图", dataset, true, false, false);PiePlot pieplot = (PiePlot) chart.getPlot();pieplot.setExplodePercent(dataset.getKey(0),0.4);pieplot.setLabelFont(new Font("宋体", 0, 12));//没有数据的时候显示的内容pieplot.setNoDataMessage("无数据显示");pieplot.setCircular(false);pieplot.setLabelGap(0.02D);String filename = ServletUtilities.saveChartAsPNG(chart, 500, 300, null, session);String graphURL = request.getContextPath() + "/DisplayChart?filename=" + filename;%><img src="<%= graphURL %>"width=500 height=300 border=0 usemap="#<%= filename %>">