15.64.56  SYMBOL 

This selects the default symbol
SET SYMBOL "xx" [SIZE=n] [THETA=n] [PHI=n] [ANGLE=n] [PERMANENT]
If no parameters are specified the symbol and size are set to NONE  and  2
and  the  angles are turned off.  In other words SET SYMBOL is the same as
SET SYMBOL=NONE SIZE=2.

This  command  determines the symbol plotted data that have no symbol.  If
you SET SYMBOL before reading data the symbol is assumed for the data  you
are reading.  See:Command READ POINTS and PLOT.