You need to create your frequency vector to plot the spectrum against, rather than just indices, as is done in examples in
doc fft
Those examples just plot a single-sided spectrum for the positive frequencies, which is usually all you want unless you especially want to do things with negative frequencies. Unless you have a complex signal the negative half of the spectrum will just mirror the positive half about either 0 or the nyquist frequency. It isn't something that is useful in most cases.
