Fix asymmetric error bars for series (closes #9536)
closes #9536

This fix is for handling asymmetric error bars for series.
It adapts to the syntax for
http://matplotlib.org/api/pyplot_api.html#matplotlib.pyplot.errorbar
where a sequence of shape 2xN is expected in case of asymmetric error bars.

If a single series is to be plotted and the error sequence is of shape 2xN
it will be used as asymmetric error bars.
Previously a 2xN error sequence was assumed to be 2 symmetric error sequences
for 2 series. Thus in the end only the first error sequence was used.
