15.11.13  Histograms 

You  may  add  together 2 histograms or average together 2 histograms with
different bin widths.  You should specify the correct DX and DY  for  each
data set you wish to add.  You probably also wish to use NORMAL=OFF

If  you do not specify FROM, TO ...  a set of values is assumed.  First if
you are binning 2 histograms with equally spaced equivalent  values,  then
the  same  X  values are assumed.  Otherwise the current X tick spacing is
used.