gnuplot_example.f90

program gnuplotexample
use, intrinsic :: iso_fortran_env, only : &
 & stdin=>input_unit, &
 & stdout=>output_unit, &
 & stderr=>error_unit
use m_process ,only: process_open_write, process_writeline
use m_process ,only: streampointer, process_close
implicit none
integer                     :: i, j
integer,parameter           :: n = 300 
real                        :: x(n) , y(n)            
type(streampointer)         :: fp
   call plotit('start')
   do j = 1 , 3000 , 1
      do i = 1 , n          ! set x() values as whole numbers 1 to n
         x(i) = i
         y(i) = sin(x(i)/25.0+j/40.0)
      enddo
      call plotit()         ! plot the values in <x,y>
   enddo
   call plotit('end')

contains

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

end program gnuplotexample