plend ends a plotting session, flushing buffers and closing the plotfile.
This routine is normally called automatically when the program exits, but must
be called when in one program run another call of plinit is used to start a
new plotting session. This may happen, for example, if one wants to print a
plot built up under X-windows.
An other application would be to make room by closing buffers and allocated
memory if plotting is ready but the program continues.