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