I have also been searching for other similar services that could generate static image charts, but have been unsuccessful so far.
However an option is to create your own "service" using php scripts on your own server, to which you can pass parameters.
You could use a php charting library, for example something like pChart to generate the graphs with php, and return the .png image from the script.
Some problems with the javascript-based charting solutions are that you normally cannot use them if you also want to generate PDF's with charts on the fly, or if you want to generate the charts inside Rich-text Editors, or just use them in emails (as you already mentioned).
