interpreting magnitude of HSPICE's FFT-output of mixed signals

prasfft

I ran a time-domain simulation in HSPICE & I am trying to analyze the frequency content of the transient current draw...(FYI- I have the power systems "impedance vs. frequency" curve, and my aim is to determine the noise voltage on my power rail given its impedance now)

So I run FFT (current-waveform,t_start, t_stop, NP) using the in-built calculator...

Now how does one interpret the resulting spectral density curve?

Is the magnitude (on Y-axis) the real amplitude of current at that frequency i.e. can I just take that amplitude and multiply it by impedance Z at that frequency to calculate the noise voltage...??

I'd highly appreciate if some one can lend insight into this...thanks a bunch