Plotting multiple columns with a for loop in gnuplot, key doesn't work -

i have file many columns i'd plot follows:

plot [i=1:30] 'test' using 1:i w lp

this gives plot want, when set key, key see has lines labeled 1:i:

enter image description here

how can make output more meaningful, displayin value of i?

if don't set explicit title, gnuplot selects automatic title based on plain plot command call. if want meaningful title, must give explicitly, like

plot [i=1:30] 'test' using 1:i w lp title sprintf("column %d", i)

Script