Got the legend layout and item size working, then went for making the legend movable. Alas, hit another bug in goocanvas. Looks like some numerical instability in goo_canvas_view_convert_to_item_space() is affecting the coordinates of motion-notify-events.
As a workaround, calling goo_canvas_view_convert_from_item_space() makes the coordinates sane again. In the end, the legend is almost working, only preserving relative position during window resizes and during load/save is to be done. Then of course there's the MathML part waiting... :P

As planned, went for some biking in the afternoon, then out dinner. In the evening, finished the legend parts described above, that had been left unfinished by the time I had to go biking.