I have data that has error bars and upper limits. (Actually they
are lower limits, since the Y axis is in stellar magnitudes and is
inverted.) My suggestion is use a negative error value to
indicate a limit in which case an arrow would be drawn, instead of an
error bar. This feature would only apply to the case of
asymmetric error bars and not to the symmetric case. I can
produce a patch if this suggestion is agreeable.
– Paul