Time Domain to Frequency domain with DSP

Question

I try to convert my time domain data to frequency domain data with Aquila DSP library.After that I calculate energy, mean and other coefficients with same library. As a sample i create a vector and Aquila::SignalSource like that

std::vector<double> xaxis = { 1,5,3,7,5,9,7,11 };
Aquila::SignalSource signalSource(xaxis);     
double energy = Aquila::energy(signalSource);

I calculate energy of that signal but i think this value for time domain.

How to calculate fft energy ?