band.txt
unset key
set ytics
set trange[-20:15]
set yrange[-2:7]
set xrange[0:2.573132]
set xtics format " "
set xtics add ("M" 0)
set xtics add ("R" 0.500000)
set xtics add ("G" 1.392341)
set xtics add ("X" 1.940458)
set xtics add ("R" 2.573132)
set parametric
plot "band.dat" index 0 using 1:2 with line,\
0.500000,t with line lt rgb "red",\
1.392341,t with line lt rgb "red",\
1.940458,t with line lt rgb "red"