GRID = expr;                Specify coordinate system for plots

The GRID control specifies the coordinate system used for subsequent plots.
GRID also affects the output produced by PRINT and WRITE.  Values for both
the implicit loop variable and expressions are transformed based on the
current grid type.

For any of the logarithmic coordinate systems, expression values must be
strictly greater than zero.  For log-log plots, the implicit loop variable
must also be strictly greater than zero.  Autoscaling is not recommended
for logarithmic plots since it frequently generates minimum values that
are less than or equal to 0 (and hence the axis lower limit will be
negative infinity).

The possible values for 'expr' are:
+
NOMINAL or NONE or SHOWAXES
Cartesian coordinates.  No conversion of the X and Y axis values before
they are plotted.

  GRID = NONE;          Suppress the fiducial tick marks within the
                        plot area.
  GRID = SHOWAXES;      Shows the X and Y axes if they appear in the
                        plot area.
  GRID = NONE+SHOWAXES; Suppresses the inner fiducial tick marks but
                        shows the X and Y axes if they appear in the
                        plot area.
+
POLAR
Polar coordinates.  X axis value (i.e.  value of the implicit loop
variable) is converted to an angle, in radians, based on the current
processing duration.  Thus, the START + BIAS value is equivalent to
0 radians and START + BIAS + DUR is equivalent to 2*pi radians.  Polar
grids have radial tick markers every 15 degrees.

Y axis value is assumed to be a magnitude value (i.e.  distance from
the origin) and thus should be greater than 0.  Y axis values less than
0 cause the point to be plotted in the quadrant 180 degrees from the
actual quadrant.  Point (0,0) is in the center of the screen.  For
example, PLOT(1) would draw a circle with a radius of one unit (it will
not actually be circular due to the aspect ratio of the plotting area
for the graphics interface).
+
POLOG
Similar to POLAR except the log base 10 of the magnitude is plotted. 
Magnitude values must be greater than or equal to 1 since log values
are negative for values less than 1 and approach negative infinity as
the magnitude value approaches 0.  Log values are undefined for
negative magnitudes.

POLN
Similar to POLOG except the log base e of the magnitude is plotted.
+
SEMILN
Logarithmic coordinates on the Y axis only.  Value plotted is the
logarithm (base e) of the Y axis value.

SEMILOG
Logarithmic coordinates on the Y axis only.  Value plotted is the
logarithm (base 10) of the Y axis value.

LN.LN
Logarithmic coordinates on both the X and Y axes.  Values plotted are
the logarithms (base e) of the X and Y axis values.

LOG.LOG
Logarithmic coordinates on both the X and Y axes.  Values plotted are
the logarithms (base 10) of the X and Y axis values.
