graphics - Obtain Data points from a graph on an image without axes

I have the following image: (which is a oscillation of the "magnetic susceptibility of gold" in y-direction, in arbitrary units, over the "magnetic field strength" from bmin = 36540 Gauss to bmax = 36784 Gauss, in x-direction).

pic = Import["http://i.stack.imgur.com/P6JOc.png"]

Even if it looks periodic, it should not be so, it must be periodic in 1/x (i.e. in the reciprocal of the field strength). Changing the image to a plot over 1/x I already succeeded in with the following Mathematica function:

bmin := 36540;


bmax := 36784;

f[a_, b_] := {1/(1/bmin - 1/bmax) (1/(a*(bmax - bmin) + bmin) - 1/bmax), b} 

img = ImageTransformation[pic, f[#[[1]], #[[2]]] &, PlotRange -> Automatic]

Now I get an image which looks quite similar and goes from 1/bmax to 1/bmin in the x-direction. From that image I now want to obtain the data points (along x) in y direction, such that I can do a Fourier Transform to obtain the frequencies and hence the periods in the reciprocal field strength.

How would I do that the easiest way?

btw: The background is that my professor wants us to obtain those frequencies in a problem set on the De-Haas-Van-Alphen-effct but obviously "by hand" with ruler and pen on a printed version of the graph...and I´m not gonna do that :)