graph not displayed in pdf

Hai,

I create a report using ireport designer 4.5.0 and insert a graph in summary part. My problem is when click on the XML button i got the source code of summary is

<summary>

<band height="164" splitType="Stretch">

<barChart>

<chart>

<reportElement x="71" y="38" width="404" height="100"/>

<chartTitle/>

<chartSubtitle/>

<chartLegend/>

</chart>

<categoryDataset>

<categorySeries>

<seriesExpression><![CDATA[""]]></seriesExpression>

<categoryExpression><![CDATA[$F{exam_details_exam_name}]]></categoryExpression>

<valueExpression><![CDATA[$F{exam_details_time}]]></valueExpression>

</categorySeries>

</categoryDataset>

<barPlot>

<plot/>

<itemLabel/>

</barPlot>

</barChart>

</band>

</summary>

 

but in jrxml source file it only displays

 

 

so i did not get the graph in my php page. when add the above code i got a blank page. Why this occure?? please give me an answer.