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