@CGForbes This is the result I get with the latest attempt - the plot is exactly right, but still no text output to transfer to excel.

@Carl Love I get "ambiguous multiple assignment" when I use this. Is there something I'm missing?

P := implicitplot([dispf, dispbeamf], kz = -150 .. 150, f3 = 0.18e11 .. 0.25e11, color = [red, blue], numpoints = 5000);
M1, M2 := map2(op, -1, [plottools:-getdata(P)])[];