FIX: workaround when geom_line gets a size mapping

Currently, matplotlib does not allow for different sizes of one line
(see https://github.com/matplotlib/matplotlib/issues/2658) so adding
a size mapping will not work (and produces an error on png devices
and does nothing on pdf devices).

Add a workaround in geom_line to detect such cases, print a warning
and set the size to a hardcoded value of "4" (can be changed by
'+ geom_line(size=x)'.

Also add updated baseline images for the corresponding tests.