Question: Plot v s. semilogplot

I have a transfer function:

xfer_mag := 1/sqrt((-1.0000 + 1.0772*10^(-14)*f^2)^2 + (1.9665*10^(-20)*f^3 - 3.6181*10^(-6)*f)^2)


if I do:  semiplotplot(20*log[10](xfer_mag),f=10..30e6) I get :


but if I do: plot(20*log[10](xfer_mag), f = 10 .. 0.30e6), and then use the menu options to the right to change the axes properties to Log, I get this:


what is going on here?  why is the magnitude not the same ?



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