Hi Jeet_rio,
Assume you have N samples at regular intervals T.
If you obtain their DFT, the separation in frequency between bins is 1/(NT).
For example: suppose you have N=256 samples taken at intervals of T=10 seconds. The total duration of the register is NT=2560 seconds.
The interval between bins is the inverse of that, i.e. 1/2560 Hz.
So, the first value of the DTF (or FFT) corresponds to frequency 0 (mean value).
The following value correspons to the component of frequency of (1/2560)Hz, the folowing to (2/2560)Hz, and so on.
Note that FFT is just an efficient way for carry on the calculation of DFT.
Regards
Z
P.D.: Sorry. I missed the part of "non-uniform", thiking that the issue was about the relationsgip between time axis scale and frequency axis scale.