If you don't mind letting the client do a bit of work I have something which allows the quick insertion and editing of graphs illustrated at http://qpr.ca/moodle (login as 'guest1' with password 'guest1' and look at the quiz in the 'Demo' course).
This isn't exactly "server side" as it employs a java applet to do the graphing, and you may not like the slight delay in presentation as the applet loads, but it does provide the kind of editing capability you asked for, in that the equations for the graphs in both the question and answers just occur as applet parameters in plain text (in standard in-line notation with ^ for powers).
Mathematics tools
server-side graph plotter ?
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