Plot data only

The `GRAPH\OVERLAY` command plots the data, but does not draw axes. Autoscaling will not apply. The `GRAPH\OVERLAY` command overlays on an existing set of axes. For example:

     GRAPH\OVERLAY X Y      ! overlay data curve on current axes
     GRAPH\HIST\OVER X Y   ! overlay histogram on current axes

If `LEGENDON` is true, i.e., non-zero, then the data variables are not necessary. That is, you can enter:

     GRAPH\OVERLAY 'legendentry'

and an entry will be made to the legend, but no curves are plotted. The plotting symbol, as set by the `PLOTSYMBOL` characteristic, will be used in the legend.

Plot axes only
Plot a smooth curve through the data