For the one who want an alternative X scale for their plot using ISIS, edit your std.gnu file according to that extract:
(take care to backup original file)
# -------------------------------------
# Setting
# -------------------------------------
set xlabel "Wavelength (A)"
set ylabel "Relative flux"
set title '$1' offset 0,-0.5
set tmargin 2
# Start of JP GODARD Modifications
set xtics out rotate by -90 3500,50 font "Courier New,7"
set xtics add ("------Halpha" 6563)
set xtics add ("------Hbeta" 4861)
set xtics add ("------Hgamma" 4341)
#set logscale y
#set ytics 1,10,100
# End of JP GODARD Modifications
set xrange [$2]
set yrange [$3]
set terminal push
