source filename as parameter

My main purpose is to exploit as data source a different file each time I invoke gnuplot from a script.

I would like to provide the source file name as a parameter.
For example:
gnuplot my_gnuplot_file data_source_filename

If it is not possible,
I would like to read from gnuplot the content of a text file where the filename is stored,
e.g., assigning to a variable the output of a cat/awk invocation.

Could you suggest me how to achieve it?

Thank you very much,
Carlo