Power Spectral Density and Fast Fourier Transform basic question
The second trace is probably the phase. You can express the spectral density either as a sine term and a cosine term (in phase and out of phase terms) or as a magnitude and phase angle.  In your first example, for your plot you have converted the sin/cos terms into a magnitude, and discarded the phase information.

Yes, log power differs from log voltage by a factor of two.  $ 2 \log{(V)} = \log{(V^2)} $. But different ways of calculating FT and inverse FT, or PSD and inverse PSD, put a constant value of $ \pi $ in different places: If you use different methods of calculation, you sometimes find that you have choosen to normalize the FT differently.