Shell, Gnuplot : Set title from variable in file

Question

I would like the title of my graphic to be a numeric variable present in a file. The same file where there are the variables that I use for the plot.

for example :

set title sprintf("Nu=f(t) avec Ra=%d",titre)

Where the titre variable is the second line of the first column of my file. I don't know how to assign this value to titre variable.

I need help, please.

Thank you