how to know the frequency on the fft plot

if I'm doing fft of my signal, and ploting it

plot(abs(fft(x)).

Now how do I match an index on the graph to a specific frequency? I came to the formula of -

f = (FPS/N)*(i-1)

f - the frequency I am looking for
FPS - the Frames Per Second of the original signal in the time domain
N - total number to recorded frames in the time domain
i - index on x-axis on my abs(fft(x)) plot.

does that correct?

Answer 1

[Walter Roberson]

Yes, except that it only works for the first half of the fft.

See http://www.mathworks.com/help/matlab/ref/fft.html and notice how they only plot the first half with a formula for "f" that works out the same as the one you have.