What about SVG, is that a possible candidate? By proposing a new format, the real challenge is to have the graphical editors to use them. Expecting users to contribute graphics by markup isn't going to be much more productive in my opinion.
Some exist, there is very simple gnuplot. There is python matplotlib but it's a general purpose programming language and no DSL specific one. I think the closest is asymptote (which is very little known). Then is is processing but it's geared toward animations and also does mostly raster graphics. There is R which is mostly for statistics. Then there is TikZ which is awful since TeX was never meant to be a programming language so it'f far from a DSL.
I want to be able to describe shapes and extend them easily. I think Scala would be wonderful and somebody fluent in it could create a DSL and still allow extreme extensibility. We need a language which has multiple layers of API. One very low which can easily do lines, bezier curves and the usual. And then some higher APIs that allow _easy_ composition and intuitive transformations.
SVG is like PDF, a format to display but nobody wants to write PDF or SVG. It's no fun.