Python, Matplotlib, subplot: How to set the axis range?

python range axis matplotlib

How can I set the y axis range of the second subplot to e.g. [0,1000] ? The FFT plot of my data (a column in a text file) results in a (inf.?) spike so that the actual data is not visible.

pylab.ylim([0,1000])

has no effect, unfortunately. This is the whole script:

# based on http://www.swharden.com/blog/2009-01-21-signal-filtering-with-python/
import numpy, scipy, pylab, random

xs = []
rawsignal = []
with open("test.dat", 'r') as f:
      for line in f:
            if line[0] != '#' and len(line) > 0:
                xs.append( int( line.split()[0] ) )
                rawsignal.append( int( line.split()[1] ) )

h, w = 3, 1
pylab.figure(figsize=(12,9))
pylab.subplots_adjust(hspace=.7)

pylab.subplot(h,w,1)
pylab.title("Signal")
pylab.plot(xs,rawsignal)

pylab.subplot(h,w,2)
pylab.title("FFT")
fft = scipy.fft(rawsignal)
#~ pylab.axis([None,None,0,1000])
pylab.ylim([0,1000])
pylab.plot(abs(fft))

pylab.savefig("SIG.png",dpi=200)
pylab.show()

Other improvements are also appreciated!

Also see stackoverflow.com/questions/15858192/…

Does this answer your question? setting y-axis limit in matplotlib

Mr. T

You have pylab.ylim:

pylab.ylim([0,1000])

Note: The command has to be executed after the plot!

Update 2021
Since the use of pylab is now strongly discouraged by matplotlib, you should instead use pyplot:

from matplotlib import pyplot as plt
plt.ylim(0, 100) 
#corresponding function for the x-axis
plt.xlim(1, 1000)

Whenever I do this, it flips the images upside down.

if i use this with hexbin, uses ylim after plot() exposes white background on both plots

WHat if you are not using plot, but savefig??

Call plot(), then ylim() and then savefig().

in subplots it's set_ylim().

ImportanceOfBeingErnest

Using axes objects is a great approach for this. It helps if you want to interact with multiple figures and sub-plots. To add and manipulate the axes objects directly:

import matplotlib.pyplot as plt
fig = plt.figure(figsize=(12,9))

signal_axes = fig.add_subplot(211)
signal_axes.plot(xs,rawsignal)

fft_axes = fig.add_subplot(212)
fft_axes.set_title("FFT")
fft_axes.set_autoscaley_on(False)
fft_axes.set_ylim([0,1000])
fft = scipy.fft(rawsignal)
fft_axes.plot(abs(fft))

plt.show()

As Rob suggests, the OO interface in matplotlib is preferred over the state-based pylab interface. "Although many examples use pylab, it is no longer recommended. For non-interactive plotting it is suggested to use pyplot to create the figures and then the OO interface for plotting." matplotlib.org/faq/…

andrewtinka

Sometimes you really want to set the axes limits before you plot the data. In that case, you can set the "autoscaling" feature of the Axes or AxesSubplot object. The functions of interest are set_autoscale_on, set_autoscalex_on, and set_autoscaley_on.

In your case, you want to freeze the y axis' limits, but allow the x axis to expand to accommodate your data. Therefore, you want to change the autoscaley_on property to False. Here is a modified version of the FFT subplot snippet from your code:

fft_axes = pylab.subplot(h,w,2)
pylab.title("FFT")
fft = scipy.fft(rawsignal)
pylab.ylim([0,1000])
fft_axes.set_autoscaley_on(False)
pylab.plot(abs(fft))

Christian Seitz

If you know the exact axis you want, then

pylab.ylim([0,1000])

works as answered previously. But if you want a more flexible axis to fit your exact data, as I did when I found this question, then set axis limit to be the length of your dataset. If your dataset is fft as in the question, then add this after your plot command:

length = (len(fft)) pylab.ylim([0,length])

Jsowa

If you have multiple subplots, i.e.

fig, ax = plt.subplots(4, 2)

You can use the same y limits for all of them. It gets limits of y ax from first plot.

plt.setp(ax, ylim=ax[0,0].get_ylim())

Python, Matplotlib, subplot: How to set the axis range?

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