program logistic_graph
      implicit none
      real r,x_ini,x,xi,y,x_plot(3),y_plot(3),fy
      integer pgopen,i,i_again,isymbol
      external fy

      write (*,*) 'This program plot xn as x_n+1 = 4rx_n(1-x_n+1).'
      write (*,*)
      write (*,*) 'What is the initial value of x (0 < x < 1) ?'
      read (*,*) x_ini

      if ( pgopen('/xwin') .le. 0 ) stop
      call pgpap(5.0,1.0)

 100  write (*,*) 'What is the value of the control parameter r ?'
      read (*,*) r

      call pgeras
      call pgsci(1)
      call pgenv(0.0, 1.0, 0.0, 1.0, 0, 0)

      call pgsci(2)
c     call pgfunx(fy,1000,0.0,1.0,1)
      do i=1,1000
        x = i/1000.0
        y = fy(x,r)
        call pgpt(1,x,y,-1)
      end do

      x_plot(1) = 0.0
      y_plot(1) = 0.0
      x_plot(2) = 1.1
      y_plot(2) = 1.1
      call pgsci(1)
      call pgline(2,x_plot,y_plot)

      call pgsci(3)
      x = x_ini
      y = 0.0
      do i=1,500
        x_plot(1) = x
        y_plot(1) = y
        x_plot(2) = x_plot(1)
        y = 4*r*x*(1-x)
        y_plot(2) = y
        y_plot(3) = y
        x_plot(3) = y
        call pgline(3,x_plot,y_plot)
        x = y
      end do

 102  write (*,*) 'Again ? (1=YES;0=NO)'
      read (*,*) i_again
      if (i_again.eq.1) goto 100
      if (i_again.eq.0) stop
      goto 102

      end


      function fy (x,r)
      real fy,x,r
      fy =  4*r*x*(1-x)
      end