after defining the this go multiview command at the top of the script you can easily make the plot. in the go multiview command there is view_name _prefix lets suppose you put it as "a". then before the fill my_var or plot my_var command you have to write "set viewport a11". Which means viewport at first row and first column.
https://www.pmel.noaa.gov/maillists/tmap/ferret_users/fu_2006/msg00501.html