Taco,
So, I guess it would make sense to test the compiler directly: what is the most basic way to trigger mplib from a plain tex file directly run through LuaTeX (i.e. the equivalent of the startMPgraphic if any? (And this might provide some more information on what's going on.)
Just look at mlib-run.lua from the context distribution, it should be clear enough.
Thanks for the pointer. I played a bit with pdfluatex directly which seems to work. (I'm not to the point of producing figures, because I would have to rebuild all the functions that are located in mlib-run.lua and the luatexref-t.tex does not give straightforward example of how the mplib module should be used, it only describes the functions API. Btw: could you add in the doc a simple example of how to set up an mplib instance (loading the format file) and typesetting a minimal graphic, and then use it from within plain TeX for instance; this is transparent to conTeXt users since there are files like mlib-run.lua but not easy for plain TeX users :P And I'm not advocating the use of plain TeX, I'm a converted conTeXt user since 2003, but this will help user understand (a bit) how things work behind the scene.) Thanks for your (and all the posters') interest, Olivier