Gnuplot 5.2.7



raise {plot_window_id}
lower {plot_window_id}

The raise and lower commands function only for a some terminal types and may depend also on your window manager and display preference settings. An example of use is shown here

set term wxt 123     # create first plot window
plot $FOO
lower                # lower the only plot window that exists so far
set term wxt 456     # create 2nd plot window may occlude the first one
plot $BAZ
raise 123            # raise first plot window 

These commands are known to be unreliable.