Fixed issues with errorbar limits

This fixes two issues with errorbar limits.  An upper limit means the exact value of a data point is unknown, but it is known not to be higher than a particular value.  This is denoted by an downward arrow whose base is at the value of the limit.  Similarly lower limits are upward arrows.  This patch corrects the arrow directions (now flipped) and makes the arrow base at the data point (the arrow is no longer straddling the data limit).  I've also included a new example file.

I think it would also be nice to add keywords to control the arrow size, face color, etc., but that is not implemented here.