Advanced
This code simply expands on the basic idea and is run as a bash file:
#!/bin/bash
awk '{print "set label",FNR, "\"",$1,$2,"\"","at", $2*3.30908",0.4 rotate"}' test.dat > show.p
awk '{print "set arrow",FNR,"from", $2*3.30908",0 to ",$2*3.30908",1.2 nohead linetype 3"}' test.dat >> show.p
When run, the bash file creates show.p that is loaded in gnuplot before plotting the Keck data file to produce:
Here is the same echelle order but using the Iodine cell and it shows the utility of marking the absorption lines:
It is easy to see that only FeII is present in these data sets.