When drawing bar charts (option bar or hbar) the bars are drawn from 0 rather

than from the minimum Y value. Bins with positive values to be drawn as bars
 extending from the X axis upwards, and bins with negative values to be drawn
as bars extending from the X axis downwards.


git-svn-id: http://root.cern.ch/svn/root/trunk@4459 27541ba8-7e3a-0410-8455-c3a389f83636