Fft - what does the root fft really count?


I have some doubts about what does the root fft really count

After Transform an instance of TH1 gets from an instance of TVirtualFFT class some complex numbers and then make the histogram which contains twice too much bins (two times more than number of samples) and which looks like 1bin column, 1bin gap, 1bin column, 1bin gap and so on.

When I saw it I put this points from TVirtualFFT using methods same like in TransformHisto function to the Graph class and then plot this numbers.
I obtained 2 separately lines which looks like one point lying on the lower line, next at the upper, next at the lowe, next at the upper and so on…

I am a student only and I do not have much experience with fourier transform, but I guess something is wrong with this numbers:
1)why we have twice too much bins (comparing with sampling theorem)
2)why we obtain two separate lines: one is lower, the second is above (but both lines looks like FT! high values at the begining and then they are getting smaller and smaller )

I guess that this points above are the upper ends of the column at histogram and the points below are the bottom ends of the column, but how are this numbers connected with the fourier transform? afaik function after FT is another function, not a histogram…

So could you explain me what numbers does the root fft actually count?

( I am talking about magnitude plot! )