15.78.4  Expression 

You  may  use  an expression inside quotes to actually calculate the value
for an existing data set.  The expression  is  evaluated  for  each  value
required.   The  lexicals  V_XVALUE,V_DXVALUE,V_ZVALUE...  are the current
X,DX,Y...  needed to perform the  calculation.   If  you  do  not  specify
NOERROR  then  the  error  is also computed when X,Y,Z are modified by the
expression.  If you use an expression you may not use  any  other  options
except for POINTS, LINES, COLUMNS, SETS, and LOG and ERROR.