even for bins with negative contents. It was not possible to get the old
  behavior.
  This functionnality correspond, in fact, to something already existing
  for 1D histograms: gStyle->SetHistMinimumZero();

  Now the lego plot behave the same way:
  If gStyle->SetHistMinimumZero() is set, the 3D boxes are drawn from 0
  even for bins with negative contents. If this option is not set the 3D
  boxes are drawn from the histogram minimum, exactly like for 1D
  histograms.


git-svn-id: http://root.cern.ch/svn/root/trunk@17495 27541ba8-7e3a-0410-8455-c3a389f83636