Here is what needs to be done
1. use the "name" variable in cfchart and type as jpg or png.
<cfchart type="jpg" name="whatever">
--- Chart stuff here ---
</cfchart>
2. Then do the following, write the entire content to the file
<cfset savedFile = getTempFile(getTempDirectory(),"Download") & ".jpg">
<cfset fileWrite(savedFile, whatever)>
3. Force the header for download
<cfheader name="Content-Disposition" value="attachment;filename=download.jpg">
<cfcontent type="image/jpeg" file="#savedFile#">
Thats ALL :-)