program logistic_graph
      real pt_x(3),pt_y(3),xl(2),yl(2)

      write(*,*)'what is the initial value of x?'
      read(*,*)x_ini
      write(*,*)'what is the value of r?'
      read(*,*)r
      
      if(pgopen('/xwin')<=0)stop
      call pgenv(0.0,1.0,0.0,1.0,1,0)
      call pgsci(2)
      dx=(1.0-0.0)/999
      do i=1,1000
        x=0.0+(i-1)*dx
        y=4*r*x*(1-x)
        call pgpt(1,x,y,-1)
      enddo
      call pgsci(1)
      call pgline(2,xl,yl)
      pt_x(1)=x_ini
      pt_y(1)=0.0
      n=1000
      call pgsci(2)
      do i=1,n
        x=pt_x(1)
        pt_x(2)=pt_x(1)
        y=4*r*x*(1-x)
        pt_x(2)=x
        pt_y(2)=y
        pt_y(3)=pt_y(2)
        pt_x(3)=pt_y(3)
        call pgline(3,pt_x,pt_y,-1)
      end do
      call pgclos
      end