PLOT (yexpr{,zexpr});  Output trace to graphics screen

The PLOT procedure uses the current scaling and labeling values to
create a plot on the graphics screen of the values calculated for
'yexpr' during the implicit loop in which this call is executed. 
Values along the X axis nominally correspond to the implicit loop
variable (i.e. step time).  Plots versus some other variable can be
accomplished via the XAXIS control statement.  In this case, the
START, BIAS, and DUR controls specify the subset of the XAXIS data file
to be used.  The number of points plotted is a function of the
duration and step size parameters.

If more than one trace is generated then each trace is labeled with
a number corresponding to the order in which the PLOT calls were
entered in the source code.  Trace numbers appear above and to
the right of their corresponding trace (unless the trace
subsequently plots through the number).  Up to 25 traces may be
accumulated on a single plot.
+
Plot scaling and labeling is determined by the control parameter
values set when the first PLOT call is executed.  All subsequent
plots are scaled accordingly until an ERASE procedure is executed. 
Trace numbers are also reset when an ERASE executes.

DAN has a simulated 3D plotting mode that is automatically invoked when
a PLOT statement contains more than one argument.  The second argument
is considered to be the Z axis value.  A left handed coordinate system
is used, so the Z axis extends into the screen.  Limited control over
the apparent viewing angle is possible with the ANGLE control
statement.  Certain restrictions apply to 3D mode; namely,

     - Use connected line segments for each trace.
     - Draw from front to back.

If these restrictions are not observed, the hidden line algorithm
used for 3D mode will not work.
