When using the errorbar(), I have no way to change the marker size (except do a
separate plot()). Should we add that to the errorbar's argument list?
Also there is another inconsistency between plot() and errorbar(), the latter
needs the fmt argument, but the former does not recognize it.
Another comment about errorbar() is that the error bar should not show in the
marker face area, i.e the error bars should be drawn first and then the markers
with no transparency.