Using title

If gnuplot is built with configuration option -enable-datastrings, then the first entry of a column of the input data file can be used as a string to provide the plot title in the key box. The column containing specified is independent of the column[s] used for the plot itself.

  plot 'data' using 1:($2/$3) title 2

In this case the entry in the first row of column 2 will be used for the key entry of the plot constructed from dividing column 2 by column 3. The entry in the first row of column 3 will be ignored.