alan,
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I don't think you can add more than what LTspice gives you already in those plots.? There is no option to change the "Quantity Plotted" on the right axis, so no, you can't "hack" it to add something else.? In .AC analysis it insists on having the left and right axes that way.
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Of course you can plot S21 and SWR in separate plot panes, which might be a better way to handle it.? ?(My LTspice is set up to use Vertical window tiles, which helps a bit.)
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Andy
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