The signals in the image represent a low pass signal and its Hilbert Transform. I know that a plot for a signal's magnitude with reference to frequencies can be found by applying a Fourier Transform. However, I've always read that the frequency plot can contain only the magnitude or only the frequency at a time. If it's only a magnitude spectrum, the values must all be positive. However, the second plot in the image has the positive frequency components in the fourth quadrant.