plotit Subroutine

subroutine plotit(step)
character(len=*),intent(in),optional :: step
character(len=10)           :: step_local
character(len=80)           :: line  
character(len=*),parameter  :: g='(*(g0,1x))'
integer                     :: ierr 
integer                     :: i
   if(present(step))then
      select case(step)
      case('start')
         call process_open_write('gnuplot --persist',fp,ierr)
         write(stdout,*)'WRITETEST: process is opened with status ',ierr
         call process_writeline( [character(len=80) :: &
          'set terminal X11',&
          'set nokey',&
          'set title " example of gnuplot data and command file generation"'],&
          fp,ierr)
      case('stop')
         call process_close(fp,ierr)
         write(*,*)'WRITETEST: process closed with status ',ierr
      end select
   else
      call process_writeline( '$set1 <<eod' ,fp,ierr)
      ierr=0
      do i = 1 , n
         write (line,g)' ', x(i) , y(i) !x! write the x,y array as coordinates to be plotted.
         call process_writeline(trim(line),fp,ierr)
         if(ierr.lt.0)then
           write(*,*)'WRITETEST: ierr is ',ierr
           exit
         endif
      enddo
      call process_writeline('eod',fp,ierr)
      call process_writeline('plot $set1 with lines lw 4',fp,ierr)
   endif
end subroutine plotit