Why matplotlib does not plot?

Question

I started to learn MatPlotLib using this tutorial for beginners. Here is the first example.

from pylab import *
X = np.linspace(-np.pi, np.pi, 256,endpoint=True)
C,S = np.cos(X), np.sin(X)

If I write these 3 lines into my python file and execute it in the command line (by typing python file_name.py), nothing happens. No error message, no plot.

Does anybody know why I do not see the plot?

ADDED

Of course I need to use show. But even if I add the following 3 lines:

plot(X,C)
plot(X,S)
show()

it still does no generate anything.

ADDED

Here are the lines that I use now:

import pylab as p
C = [1,2,3,4]
S = [10, 20, 30, 10]
p.plot(C,S)
p.show()

I still have the same result (nothing).

@Roman are you using Ubuntu 12.10? For me everything was working on ubuntu 12.04 but as soon as I moved to 12.10 everything, plots stopped showing up. I am assuming its a distro related issue in my case at-least. savefig(filename) still works though. — Shashank Singh
– Shashank Singh, Commented Jan 28, 2013 at 9:29

auselen · Accepted Answer · 2014-04-07 08:13:55Z

55

It could be a problem with the backend. What is the output of python -c 'import matplotlib; import matplotlib.pyplot; print(matplotlib.backends.backend)'?

If it is the 'agg' backend, what you see is the expected behaviour as it is a non-interactive backend that does not show anything to the screen, but work with plt.savefig(...). You should switch to, e.g., TkAgg or Qt4Agg to be able to use show. You can do it in the matplotlib.rc file.

@shashank: I run matplotlib both on 12.04 and 12.10 without problems. In both cases I use the Qt4Agg backend. If you don't have the matplotlibrc set, the default backend is used. I'm sure that for Precise matplotlib repo was built with TkAgg. If the Quantal version has been built with e.g. Agg, then that would explain the difference

edited Apr 7, 2014 at 8:13

auselen

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answered Jan 28, 2013 at 10:15

Francesco Montesano

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9 Comments

Roman Over a year ago

thank you for the answer. Now I use plt.savefig('name.png') and os.system('eog name.png &') and get what I need. To be honest I do not know what "the 'agg' backend" means, but now I have everything what I need (the working solution). Thanks.

Francesco Montesano Over a year ago

here you can find an explanation of what a backend is (sec 1.3). Your solution looks overly complicated when you have an in build mechanism to show the figure. Give a look to this page to learn about the matplotlibrc and how does it work. The best option would be to dump it into ~/.matplotlib/ and play with the backends untill you don't get show working (it's the first entry in the matplotlibrc file).

stephenbez Over a year ago

You can find where the matplotlibrc file is by running: import matplotlib matplotlib.matplotlib_fname() See more here: matplotlib.org/users/customizing.html

lv10 Over a year ago

Using 14.04 Ubuntu was able to fix the problem using Qt4Agg.

amc Over a year ago

And why not just use matplotlib.use('Agg') at the beginning?

|

oz123 · Accepted Answer · 2013-01-28 09:22:39Z

14

You need to call the function:

show()

to be more exact:

pylab.show()

and even better don't use:

from pylab import *

rather do:

import pylab as p:

and then:

X = np.linspace(-np.pi, np.pi, 256,endpoint=True)
C,S = np.cos(X), np.sin(X)

p.plot(C,S)
p.show()

answered Jan 28, 2013 at 9:22

oz123

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1 Comment

Roman Over a year ago

sorry my question was incomplete (I have edited it). Even if I use show, it does not produce anything.

Siddharth Naik · Accepted Answer · 2017-10-10 07:01:24Z

7

Try adding. I use Jupyter and this worked for me.

  %matplotlib inline

answered Oct 10, 2017 at 7:01

Siddharth Naik

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