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