Recently, I studied a paper called "What Does Your Chair Know About Your Stress Level?
It can be download at the link below.
at page 210, Fig.5 (picture 5) mentioned that
Spectra of the norm of the CoP vector during the experiment (stages 3–7) for the same subject used in Fig. 4.
How did they do to transform Fig.4 to Fig.5? And what do x-axis and y-axis mean in Fig.5?