A example to show how to plot spectrum for given signal.

plot_spec.py

# -*- coding: utf-8 -*-
"""
Plot spectrum of given signal.
"""

from numpy import sin, linspace, pi
from pylab import plot, show, title, xlabel, ylabel, subplot
from scipy import fft, arange

def plot_spectrum(y, freq_sampling):
    """
    Plosampling_itvl a Single-Sided Amplitude Spectrum of y(t)
    """
    n = len(y) # length of the signal
    k = arange(n)
    T = n / freq_sampling
    frq = k/T # two sides frequency range
    frq = frq[range(int(n/2))] # one side frequency range
    
    Y = fft(y) / n # fft computing and normalization
    Y = Y[range(int(n/2))]
     
    plot(frq,abs(Y),'r') # plotting the spectrum
    xlabel('Freq (Hz)')
    ylabel('|Y(power)|')

freq_sampling = 150.0  # sampling rate
sampling_itvl = 1.0 / freq_sampling # sampling interval
t = arange(0, 1, sampling_itvl) # time vector

ff = 5  # frequency of the signal
y = sin(2 * pi * ff * t)

subplot(2, 1, 1)
plot(t, y)
xlabel('Time')
ylabel('Amplitude')
subplot(2, 1, 2)
plot_spectrum(y, freq_sampling)
show()