[Gretl-users] How to specify a file name that uses variables from a list

I have a list of variables {alpha, beta, gamma, delta} and want to plot each of them against a variable called "time" and print the gnuplot to a file using the ".png" option. How do I rename each file during the loop so that they do not overwrite each other. I would like to print to files labeled alpha.png, beta.png, gamma.png, and delta.png. This is the code that I have. The list name is alphabet. The questions marks are the part of the code I can't figure out. loop foreach letter alphabet gnuplot $letter time --output= ????????.png endloop