The handle of a 'main' figure to be
positioned at the top-left side of the screen, which is
supposed to be the position attracting first the attention
of a user.
- If plmain not given, or it is empty, it is set to be
the smaller handle, which normally is the handle of the
first created figure.
- If plmain is set to zero (0), then function cascade is
applied.
Data Types: single| double