TRACENUM = expr;             Enable/disable trace numbers

TRACENUM enables and disables trace numbering on plots.  When
symbols are used to identify traces on a plot, the trace numbers
are frequently superfluous and may add too much clutter to the
plot.  In such a case the trace numbers may be turned off.

Trace numbers can be applied to one trace but not to another by
grouping the PLOT and TRACENUM statements.  For example,

      TRACENUM=OFF & PLOT(x*x) &
      TRACENUM=ON & PLOT(x*x*x + 2*x*x + 3);

Even though a trace may have no trace number applied to it, the
trace still reserves its own number, and consequently, there may be
gaps in the trace numbers if at least one trace has had its
numbering disabled.  In the above example the trace that is
numbered is given trace number 2.
