15.8.11  Notes 

For  mesh  data  the  independent  coordinate must be Z.  For 2-d data the
independent coordinate is always Y.  If you add normal data to  mesh  data
each  normal  Y value is added to the corresponding mesh Z value.  You may
not add mesh data to normal 2-d data.

If  EFFICIENCY  is  not  specified then the new value is Y3=Y1(+-)Y2.  The
errors  (DY)  are  treated  as  if  the   data   sets   are   uncorrelated
DY3=SQRT(DY1**2+DY2**2).

The ADD command with CHECK=OFF allows you to combine X,Y,Z...  data
     TD:SWAP  X  Y SET=1 from 1 data set with mismatched data from another
set.  For example you wish to plot Y of set 1 vs Y of set 2.
     TD:SWAP X Y SET=1
     TD:ADD 1,1 APPEND (Produces set 3=set 1)
     TD:Y=0 SET=LAST (Set 3 has only X)
     TD:ADD 2,3 CHECK=OFF (Set 3 has Y(1),Y(2))
Set 3 now has X=Y(set 1) and Y=Y(set 2)