subroutine tsp_display ( prefix, n, x2 )

c*********************************************************************72
c
cc tsp_display() plots cities and a tour connecting them.
c
c  Discussion:
c
c    If a "round trip" is desired, then the x2 array must repeat the
c    first city at the end.
c
c  Licensing:
c
c    This code is distributed under the MIT license.
c
c  Modified:
c
c    12 June 2026
c
c  Author:
c
c    John Burkardt
c
c  Input:
c
c    character ( len = * ) prefix: a string that defines the name of
c    the problem.  It is also used to create file names needed for
c    input to gnuplot.
c
c    integer n: the number of cities.
c
c    double precision x(n,2): the x and y coordinates of the cities.
c
      implicit none

      integer n

      character ( len = 80 ) command_filename
      integer command_unit
      character ( len = 80 ) data_filename
      integer data_unit
      integer i
      character ( len = * ) prefix
      double precision x2(n,2)
c
c  Create the data file.
c
      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
      close ( unit = data_unit )
      write ( *, '(a)' ) '  Created data file "' 
     &  // trim ( data_filename ) // '".'
c
c  Create command file.
c
      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 pt 7 ps 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 )

c*********************************************************************72
c
cc get_unit() returns a free Fortran unit number.
c
c  Discussion:
c
c    A "free" Fortran unit number is a value between 1 and 99 which
c    is not currently associated with an I/O device.  A free Fortran unit
c    number is needed in order to open a file with the OPEN command.
c
c    If IUNIT = 0, then no free Fortran unit could be found, although
c    all 99 units were checked (except for units 5, 6 and 9, which
c    are commonly reserved for console I/O).
c
c    Otherwise, IUNIT is a value between 1 and 99, representing a
c    free Fortran unit.  Note that GET_UNIT assumes that units 5 and 6
c    are special, and will never return those values.
c
c  Licensing:
c
c    This code is distributed under the MIT license.
c
c  Modified:
c
c    26 October 2008
c
c  Author:
c
c    John Burkardt
c
c  Output:
c
c    integer IUNIT, the free unit number.
c
      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