在virtualenv环境下使用matplotlib绘图时遇到了这样的问题：
>>> import matplotlib.pyplot as plt
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
...
in <module>
from matplotlib.backends import _macosx
RuntimeError: Python is not installed as a framework. The Mac OS X backend will not be able to function correctly if Python is not installed as a framework. See the Python documentation for more information on installing Python as a framework on Mac OS X. Please either reinstall Python as a framework, or try one of the other backends. If you are Working with Matplotlib in a virtual enviroment see 'Working with Matplotlib in Virtual environments' in the Matplotlib FAQ
似乎是因为虚拟环境与默认环境的安装配置不同造成的。
搜索错误信息之后，在STO上找到了解决方案：
1、pip安装matplotlib之后，会在根目录下产生一个.matplotlib的目录:
➜ bin ll ~/.matplotlib
total 280
-rw-r--r-- 1 me staff 78K 10 4 2015 fontList.cache
-rw-r--r-- 1 me staff 59K 1 17 15:56 fontList.py3k.cache
drwxr-xr-x 2 me staff 68B 10 4 2015 tex.cache
2、在这个目录下创建一个名为matplotlibrc的文件，内容是：
backend: TkAgg
然后保存退出，重启Python交互界面或重新运行脚本，import正常执行。
STO答案地址：http://stackoverflow.com/questions/21784641/installation-issue-with-matplotlib-python