i have successfully captured a snapshot of a fusionchart on a headless linux server. pseudo-steps:
install xvfb and firefox
configure firefox to automatically save images in some well known directory
run Xvfb on an unused DISPLAY and run firefox on it
run "firefox -no-remote $URL" where URL hits a page that uses the "function FC_Rendered(DOMId){ myChartId.saveAsImage(); }" hook to automatically save the image once the chart is rendered
poll the saved image directory to look for any new files
profit
