That is not the question you asked.
If you need to do a harmonic analysis (Fourier transform?), then you should not use emonLib. You will need to read the analogue values directly from the ADC, for every ADC that I am aware of, each of those numbers will be the instantaneous value when you instructed the ADC to sample the input.
You will need to write the software to process those numbers. Also, you should probably not use the Arduino analogRead( ) function because it is very slow, you will read the samples much faster - meaning better accuracy and higher harmonics - if you set the ADC to sample continuously and use interrupts to read the ADC registers directly. The Atmel data sheet for the ATMega 328P has all the details, you might also like to look at emonLibCM as that uses the same technique (but like emonLib, that too only produces rms average values for voltage and current, and the average real power, but over the whole reporting interval, not just a 200 ms sample, which is typically how emonLib is used).