Hi Janet,
When you run the XPage on Notes client it adds /xsp/ in the path of XJFC.xsp (this XPage generates the chart image). So your <IMG> tag in Notes client looks something like this:
<img id="view:_id1:_id2:imgChart" src="/xsp/Naveen/JavaCharts.nsf/xsp/XJFC.xsp?chartID=sngleifw1beo&type=pie">
In your error message you get like this:
com.ibm.designer.runtime.domino.adapter.util.PageNotFoundException: Could not create the page /xsp/XJFC.xsp because the class xsp.xsp.XJFC could not be found.
At that path (/xsp/XJFC.xsp) there is no XPage so the error. I don't know why Notes client does it that way and I even raised this question in XPages forum and they suggested me to use a normal <IMG> tag. In case of browser it works because it does not add anything to it.
Regarding the PDF generation, the reason it works in Notes client is because in my JavaChartsCSJS.js file I explicitly call the XPDF.xsp (this XPage generates the PDF file) thus preventing Notes client from poking its nose in it.
Now coming to the solution. What I have done is to create a computed field which will generate the <IMG> tag that will be shown in Notes client while the current <xp:image> tag will be shown in browser.
For this go to the Custom Control, say, PieChart. There you will find a code snippet:
<xp:image id="imgChart">
<xp:this.url><![CDATA[#{javascript:"XJFC.xsp?chartID=" + compositeData.chartID + "&type=pie"}]]></xp:this.url>
</xp:image>
Replace the above code with this:
<span style="color:#008000;"><xp:image id="imgChart"></span></div>
<span style="color:#008000;"><xp:this.url><![CDATA[#{javascript:"XJFC.xsp?chartID=" + compositeData.chartID + "&type=pie"}]]></xp:this.url></span></div>
<span style="color:#008000;"><xp:this.rendered><![CDATA[#{javascript:@ClientType() != "Notes"}]]></xp:this.rendered></span></div>
<span style="color:#008000;"></xp:image></span></div>
<span style="color:#008000;"><xp:text escape="false" disableTheme="true"></span></div>
<span style="color:#008000;"><xp:this.value><![CDATA[#{javascript:"<img src=\"XJFC.xsp?chartID=" + compositeData.chartID + "&type=pie\">"}]]></xp:this.value></span></div>
<span style="color:#008000;"><xp:this.rendered><![CDATA[#{javascript:@ClientType() != "Web"}]]></xp:this.rendered></span></div>
<span style="color:#008000;"></xp:text></span></div>
This should do the trick. Do let me know if this works and if it does then I will surely include it in Java Charts v1.3 
Also, if you want it for other charts then you will have update the code of BALChart, GanttChart, HistogramChart & ScatterChart accordingly (with minor tweaks).
Regards,
Naveen Maurya