15.40.18  Limits 

The  limits  of  the  data  for  histograms  is  normally  set so that the
lowerlimit of the dependent axis with a linear scale is 0.0,  unless  some
data points are less than zero.  If you wish a different value you can:
     TD:SET LIMIT YMIN=n
          or for 3-d....
     TD:SET LIMIT ZMIN=n


                                  WARNING

     If  this  command  is  issued in between 2 plots overlaying each
     other, they will not have the same scales.