I have not seen a free site that will fit your need.

There are some inexpensive program packages available (probably at your local computer store).

Just wondering:

How was the graph generated?

How did you get the data points?

If you scaled the data points from the image, you will have a small error introduced into each data point; the resulting equation will not give an exact replica of your original.

If may be very close -- is that good enough?

If you have the data points, it should not be that difficult to fit a fourier series to it.

How many points in your .csv or .txt file?