Graph legend

If `LEGENDON` is set true, i.e., non-zero, a legend entry is drawn into a legend frame box. A legend entry consists of a short line segment, with or without plotting symbol(s), and a string. The legend entry is drawn when the `GRAPH` command is entered. The string portion of the legend entry is expected as the first parameter of the `GRAPH` command. For example:

     GRAPH 'legend entry' X Y

If `LEGENDON` is false, i.e., zero, a string entered as a first parameter with the `GRAPH` command will be ignored.

Plotting symbols
Axis scaling