subroutine tsp_display ( prefix, n, x2 )

!*****************************************************************************80
!
!! tsp_display() plots cities and a tour connecting them.
!
!  Discussion:
!
!    The code automatically adds a final return to the first city.
!
!  Licensing:
!
!    This code is distributed under the MIT license.
!
!  Modified:
!
!    28 July 2026
!
!  Author:
!
!    John Burkardt
!
!  Input:
!
!    character ( len = * ) prefix: a string that defines the name of
!    the problem.  It is also used to create file names needed for
!    input to gnuplot.
!
!    integer n: the number of cities.
!
!    real ( kind = rk8 ) x(n,2): the x and y coordinates of the cities.
!
  implicit none

  integer, parameter :: rk8 = kind ( 1.0D+00 )
  integer n

  character ( len = 80 ) command_filename
  integer command_unit
  character ( len = 80 ) data_filename
  integer data_unit
  integer i
  character ( len = * ) prefix
  real ( kind = rk8 ) x2(n,2)
!
!  Create the data file.
!
  call get_unit ( data_unit )
  data_filename = prefix // '_data.txt'
  open ( unit = data_unit, file = data_filename, status = 'replace' )
  do i = 1, n
    write ( data_unit, '(2x,g14.6,2x,g14.6)' ) x2(i,1), x2(i,2)
  end do
  i = 1
  write ( data_unit, '(2x,g14.6,2x,g14.6)' ) x2(i,1), x2(i,2)
  close ( unit = data_unit )
  write ( *, '(a)' ) '  Created data file "' // trim ( data_filename ) // '".'
!
!  Create command file.
!
  call get_unit ( command_unit )
  command_filename = prefix // '_commands.txt'
  open ( unit = command_unit, file = command_filename, status = 'replace' )
  write ( command_unit, '(a)' ) '# ' // trim ( command_filename )
  write ( command_unit, '(a)' ) '#'
  write ( command_unit, '(a)' ) '# Usage:'
  write ( command_unit, '(a)' ) '#  gnuplot < ' // trim ( command_filename )
  write ( command_unit, '(a)' ) '#'
  write ( command_unit, '(a)' ) 'set term png'
  write ( command_unit, '(a)' ) 'set output "' // prefix // '.png"'
  write ( command_unit, '(a)' ) 'set xlabel "<-- X -->"'
  write ( command_unit, '(a)' ) 'set ylabel "<-- Y -->"'
  write ( command_unit, '(a)' ) 'set title "' // prefix // '"'
  write ( command_unit, '(a)' ) 'set grid'
  write ( command_unit, '(a)' ) 'unset key'
  write ( command_unit, '(a)' ) 'set style data lines'
  write ( command_unit, '(a)' ) 'plot "' // trim ( data_filename ) // &
    '" using 1:2 lw 3 linecolor rgb "blue", \'
  write ( command_unit, '(a)' ) '     "' // trim ( data_filename ) // &
    '" using 1:2 with points pointtype 7 pointsize 3 linecolor rgb "green"'
  write ( command_unit, '(a)' ) 'quit'
  close ( unit = command_unit )
  write ( *, '(a)' ) &
    '  Created command file "' // trim ( command_filename ) // '".'

  return
end
subroutine get_unit ( iunit )

!*****************************************************************************80
!
!! get_unit() returns a free Fortran unit number.
!
!  Discussion:
!
!    A "free" Fortran unit number is a value between 1 and 99 which
!    is not currently associated with an I/O device.  A free Fortran unit
!    number is needed in order to open a file with the OPEN command.
!
!    If IUNIT = 0, then no free Fortran unit could be found, although
!    all 99 units were checked (except for units 5, 6 and 9, which
!    are commonly reserved for console I/O).
!
!    Otherwise, IUNIT is a value between 1 and 99, representing a
!    free Fortran unit.  Note that GET_UNIT assumes that units 5 and 6
!    are special, and will never return those values.
!
!  Licensing:
!
!    This code is distributed under the MIT license.
!
!  Modified:
!
!    26 October 2008
!
!  Author:
!
!    John Burkardt
!
!  Output:
!
!    integer IUNIT, the free unit number.
!
  implicit none

  integer i
  integer ios
  integer iunit
  logical lopen

  iunit = 0

  do i = 1, 99

    if ( i /= 5 .and. i /= 6 .and. i /= 9 ) then

      inquire ( unit = i, opened = lopen, iostat = ios )

      if ( ios == 0 ) then
        if ( .not. lopen ) then
          iunit = i
          return
        end if
      end if

    end if

  end do

  return
end