Syntax: POINTS pexpr pexpr [ IF (expr) ]

   (`Pexpr' is the name of a vector, or an expression in parentheses,
e.g. POINTS x (lg(y)) )

   POINTS makes points of the current style (PTYPE), linetype (LTYPE),
colour (CTYPE), size (EXPAND), and rotation (ANGLE) at the points in
vectors WORD1 and WORD2.  If the IF clause is present, only use those
points for which expr (see ARITHMETIC) is non-zero.