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this is my first question here on stackoverflow and I hope I will not make huge mistakes. I am analyzing a set of time series with sampling rate of 1 Hz. I need to plot their fourier transform in order to study their spectra. Here it is my piece of code: from obspy.core import read import numpy as np import matplotlib.pyplot as plt st = read('../SC_noise/*HEC_109C*_s', format='SAC') stp = st.copy() stp.detrend('linear') stp.taper('cosine') for tr in stp: dataonly = tr.data spec = np.fft.rfft(dataonly) plt.plot(abs(spec)) plt.show() This works just fine: the plot is the same I get using SAC. But the xaxis does not show frequencies. I've wandered around a little bit and found different ideas: none of them is working. For example in the case of a fft (here I am using a rfft) this should do the job samp_rate=1 freq = np.fft.fftfreq(len(spec), d=1./samp_rate) But if I use it it would give me negative frequencies. Does anybody have an idea? Thank you very much in advance for all the help! Piero

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