Help System (web edition)

GRID=<X or Y or XY> 
(in PLOT operation) draws straight line parallel to X axis (GRID=X)
or Y axis (GRID=Y) or both (GRID=XY) through the points indicated by
XSCALE and YSCALE, respectively.
GRID=<x step>,<y step> 
works as GRID above, but uses the steps <x step> and <y step> in
drawing. If a step parameter is negative, only a small tick is
drawn (outside the inner frame).
Mixed notations like GRID=0.1,Y are also possible
  P = more information on plotting