Syntax: FORMAT x-format-string y-format-string

   Allow the user to specify the axis tick label formats. The format
should be given as a standard C (e.g. %.2g) or Fortran (e.g. F10.4)
format. This format will be in effect until unset by issuing the
FORMAT command with no argument, in which case SM will figure out the
best format for you, or by issuing a new FORMAT command with new
format strings.

   If the format-string is ``0'', no change will be made to that
format; if it is ``1'', the default will be reinstated for that axis.
(So FORMAT with no arguments is equivalent to `FORMAT 1 1').