sudo apt-get install python-matplotlib

$ python2
Python 2.7.3 (default, Feb 27 2014, 20:00:17)
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import matplotlib
>>>

$ python3
Python 3.2.3 (default, Feb 27 2014, 21:33:50)
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import matplotlib
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named matplotlib
>>>

$ python
bash: /usr/bin/python: No such file or directory

sudo apt-get install python3-matplotlib