Reported in: http://permalink.gmane.org/gmane.comp.python.matplotlib.general/32019

In the pdf backend, draw_path_collection contains

    filled = len(facecolors)
    stroked = len(edgecolors)

so it fills if any fill color is specified, and strokes if any stroke color is specified. This is not the same logic as for other objects. Specifically, a linewidth of zero should cause the paths not to be stroked (because in pdf, a zero-width line is a line of some device-specific minimum width, whereas in agg a zero-width line is a line not drawn).

I think the code needs to make the filled/stroked decisions when creating the XObjects, so the current _iter_collection_raw_paths method is not quite sufficient.