strut2+jfreechart+mysql页面的图形无法显示

我用strut2+jfreechart+mysql生成折线图（jfreechart和plugin都导入到lib目录下了），页面无法显示折线图，当程序也没报错。页面显示的是一个叉叉，不知道是什么原因。代码如下：
struts.xml配置文件为：

<package name="chart" extends="jfreechart-default">

  <action name="displayChart"  class="wit.cl.action.DisplayChartAction">
     <result name="success" type="chart">

        <param name="width">700</param>
        <param name="height">400</param>
     </result>
  </action>
</package>

action的代码如下：(数据集的数据是从数据库取出来的，用的是hibernate,数据表很简单，横坐标是时间，纵坐标是位移)

package wit.cl.action;

import java.awt.Color;
import java.awt.Font;
import java.io.IOException;
import java.io.PrintWriter;
import java.text.DecimalFormat;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.List;

import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import org.jfree.chart.ChartFactory;
import org.jfree.chart.ChartRenderingInfo;
import org.jfree.chart.ChartUtilities;
import org.jfree.chart.JFreeChart;
import org.jfree.chart.axis.CategoryAxis;
import org.jfree.chart.axis.NumberAxis;
import org.jfree.chart.axis.NumberTickUnit;
import org.jfree.chart.axis.ValueAxis;
import org.jfree.chart.entity.StandardEntityCollection;
import org.jfree.chart.plot.CategoryPlot;
import org.jfree.chart.plot.PlotOrientation;
import org.jfree.chart.servlet.ServletUtilities;
import org.jfree.chart.title.LegendTitle;
import org.jfree.chart.title.TextTitle;
import org.jfree.data.Range;
import org.jfree.data.category.DefaultCategoryDataset;

import wit.cl.model.Wittable1;
import wit.cl.service.Wittable1Service;
import wit.cl.service.impl.Wittable1ServiceImpl;

import com.opensymphony.xwork2.ActionContext;
import com.opensymphony.xwork2.ActionSupport;

public class DisplayChartAction extends ActionSupport {
     Wittable1Service  service=new Wittable1ServiceImpl();
    private JFreeChart chart;
    public String execute()throws Exception
    {

     chart = createChart(createDataset(service.findAll()));
     return "success";

    }
    //创建数据集对象
    private  DefaultCategoryDataset createDataset(List<Wittable1> list) {
DefaultCategoryDataset defaultCategoryDataset = new DefaultCategoryDataset();
    String s = "";
if (!list.isEmpty()) {
for (int i = 0; i < list.size(); i++) {
Wittable1 w = list.get(i);
String text = "2011-1-1";
Date d = w.getDate();
System.out.println("位移数据"+w.getMove());
if (d != null) {
SimpleDateFormat f = new SimpleDateFormat("yyyy/MM/dd");
text =  f.format(d);
}
defaultCategoryDataset.addValue(w.getMove(), s,text);
}
}

return defaultCategoryDataset;
}
    //生成JFreeChart对象
private  JFreeChart createChart(DefaultCategoryDataset defaultCategoryDataset) {
JFreeChart jfreechart = ChartFactory.createLineChart("图1-1-1 左岸坝基1041m高程0+59.4坝纵多点位移计M41LBP绝对位移过程线",
"日期", "位移", defaultCategoryDataset,  PlotOrientation.VERTICAL, true, true, false);
jfreechart.setBackgroundPaint(Color.white);//设置背景颜色
//TextTitle t=new TextTitle();
//对折线图进行标绘的类CategoryPlot
CategoryPlot categoryplot = jfreechart.getCategoryPlot();
        categoryplot.setBackgroundPaint(Color.white);
        categoryplot.setDomainGridlinePaint(Color.white);
        categoryplot.setDomainGridlinesVisible(true);

Font font = new Font("宋体", Font.BOLD, 20);
    //设置标题字体格式
    TextTitle t =  jfreechart.getTitle();
    t.setFont(font);

    //设置下标字体格式
    LegendTitle l = jfreechart.getLegend();
    l.setItemFont(font);
    //设置x轴字体格式

    CategoryAxis x =  categoryplot.getDomainAxis();
    x.setTickLabelFont(font);
    x.setLabelFont(font);
  //设置y轴字体格式
    ValueAxis y =  categoryplot.getRangeAxis();
    NumberAxis ny = (NumberAxis) y;
    y.setTickLabelFont(font);
    y.setLabelFont(font);
    //y.setTickMarkInsideLength( );
    ny.setRange(new Range(-1d,5));//y轴最大最小刻度值
    //设置y轴刻度
    ny.setTickUnit(new NumberTickUnit(1.0d));//最小刻度跨度设置为1

  ny.setAutoRangeIncludesZero(true);
      DecimalFormat df = new DecimalFormat("#0.0");
      ny.setNumberFormatOverride(df);

    //lineandshaperenderer.setBaseShapesVisible(true);
    categoryplot.setDomainGridlinesVisible(true);
    categoryplot.setRangeGridlinePaint(Color.blue);
return jfreechart;
}

}
页面的代码如下：
<img src="chart/displayChart"/>
但是最后页面显示的确实一个红色的叉叉，不知道是什么问题，请大侠们指教！ --------------------编程问答-------------------- 楼主看看你的配置文件，以前遇到过配置中有问题会导致显示问题的 --------------------编程问答-------------------- debug呀　　Wittable1Service service=new Wittable1ServiceImpl();

service有值吗

补充：Java ,  Web 开发