Problems while plotting on Matplotlib

Question

I'm having a problem using python matplotlib while creating a basic plot of a function, in wolfram alpha and other plotting engines the final plot seems to be so different from the one I'm creating via matplotlib.

I followed the example inside matplotlib and I just replaced the np.sin(x) function for the function I need to plot.

I'm using several functions so this is the first one I need to plot but it's not working at all.

Here's the code I'm using and the plot comparison just ahead.

__author__ = 'alberto'
#
import numpy 
import matplotlib.pyplot as plt
#
x = numpy.arange(-20, 20, 0.1)
y = ((3*x**2) + 12/(x**3) - 5)
plt.plot(x, y)
plt.show()

Wolfram

Function plotted by wolfram

Matplotlib.

Function plotted by matplotlib

I'm using Anaconda Python(2.7.8).

Have a nice day!!!

unutbu · Accepted Answer · 2015-01-12 18:49:44Z

3

You can make one call to plt.plot generate two disconnected curves (thus handling asymptotes) by assigning nan to extreme values. Mathematica handles this for you automatically; matplotlib requires you to do a little work:

import numpy  as np
import matplotlib.pyplot as plt

x = np.linspace(-20, 20, 1000)
y = ((3*x**2) + 12/(x**3) - 5)
mask = np.abs(y) > 100
y[mask] = np.nan
plt.plot(x, y)
plt.grid()
plt.show()

yields

enter image description here

answered Jan 12, 2015 at 18:49

unutbu

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1 Comment

Juan Reséndiz Over a year ago

Thanks you all for your answers! Finally I got what I was doing, Thanks a lot! I was smashing my head into the keyboard X__X

BrenBarn · Accepted Answer · 2015-01-12 18:46:21Z

1

If you look at the axis labels, you can see that matplotlib is showing you a much larger viewing window than Wolfram Alpha. Wolfram alpha is showing you roughly -4 <= x <= 4 and -50 <= y <= 100, but matplotlib is showing you -20 <= x <= 20 and y limits that are gigantic.

To get a comparable graph, set the view limits:

plt.xlim(-4, 4)
plt.ylim(-50, 100)

answered Jan 12, 2015 at 18:46

BrenBarn

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Comments

David Zwicker · Accepted Answer · 2015-01-12 18:44:16Z

0

You have to restrict the vertical dimension manually! Here is a possible solution:

import numpy 
import matplotlib.pyplot as plt

x = numpy.arange(-20, 20, 0.1)
y = ((3*x**2) + 12/(x**3) - 5)
plt.plot(x, y)
plt.ylim(-50, 100)
plt.show()

Note the plt.ylim function!

answered Jan 12, 2015 at 18:44

David Zwicker

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