求助！关于jpgraph

浏览器为何没有图像输出，代码如下：
<?php
require_once( 'src/jpgraph.php');
require_once ('src/jpgraph_bar.php');        //导入Jpgraph类库的柱状图功能
$data = array(80,73,89,85,92,75,85,65,84,79,85,98);          //设置统计数据
$graph = new Graph(600,300);   //设置画布大小                                                  
$graph->SetScale('textlin');   //设置坐标刻度类型                                                          
$graph->SetShadow();    //设置画布阴影                                                                
$graph->img->SetMargin(40,30,20,40);       //设置统计图边距
$barplot = new BarPlot($data);        //实例化BarPlot对象                                               
$barplot->SetFillColor('blue');          //设置柱形图前景色                                              
$barplot->value->Show();
$graph->Add($barplot);
$graph->title->Set('吉林省***科技有限公司年度收支');            //设置统计图标题
$graph->xaxis->title->Set('月份');                                                  //设置X轴名称
$graph->yaxis->title->Set('总金额(万元)');        //设置Y轴名称
$graph->title->SetFont(FF_SIMSUN, FS_BOLD);       //设置标题字体
$graph->xaxis->title->SetFont(FF_SIMSUN,FS_BOLD);           //设置X轴字体
$graph->yaxis->title->SetFont(FF_SIMSUN,FS_BOLD);           //设置Y轴字体
    $graph->Stroke();     //输出图像
?> 
运行之后，浏览器显示一个叉号而没有图像的输出，这是怎么回事？谢谢