gnuplot> plot [:] [17:12] "hr.dat" using ($5-$4-0.8):($4-3.7)
gnuplot> set xlabel "B-V"
gnuplot> set ylabel "V"
gnuplot> replot
gnuplot> set terminal png
Terminal type set to 'png'
Options are 'small color picsize 640 480 '
gnuplot> set output "hr.png"
gnuplot> replot