This demo shows that it is possible to edit your XML in a user-friendly manner, while updating an SVG chart at the same time. Check an the alpha demo of Xopus in Firefox. It really is alpha quality with a lot of known bugs, so please don't report any bug unless it won't run at all. In the demo you can see a pie chart which is drawn using SVG. Every change in the XML updates the pie chart in real time. Besides SVG output, MathML is also supported.