How to interpret the image made by using FFT function directly

For this kind of picture, I'd like to know what it means. I just import a column of data into Matlab, make this column as X, and then directly use y = FFT (x); plot (y) to get the image

Answer 1

[Ameer Hamza]

Generally, fft of a real-values vector contain complex values. When you directly use

y = fft(x)
plot(y)

It just plots real components (x-axis) against complex components (y-axis). I am not sure if this gives any useful information. The usual way is to plot the abs() of the fft vector which gives information about frequency response.

y = fft(x)
plot(abs(y))

See the first example here: https://www.mathworks.com/help/matlab/ref/fft.html