At present, the plot option window is ignored when gnuplot_term is anything other than default. It would be great to allow the window number to be pasted into a non-default terminal description string.
One possible mechanism is to recognize some notation such as {$window} or ${window} or {window} or \(window) or whatever.
I don't think it makes sense to just have ~d or %d or whatever, since that makes the results dependent on the number and order of pasted-in variables.