python matplotlib framework under macosx?
Simply aliasing a new command to launch python in ~/.bash_profile will do the trick.
alias vpython3=/Library/Frameworks/Python.framework/Versions/3.6(replace with your own python version)/bin/python3
then 'source ~/.bash_profile' and use vpython3 to launch python3.
Explanation: Python is actually by default installed as framework on mac, but using virtualenv would link your python3 command under the created virtual environment, instead of the above framework directory ('which python3' in terminal and you'll see that). Perhaps Matplotlib has to find the bin/ include/ lib/,etc in the python framework.
I am getting this error:

/sw/lib/python2.7/site-packages/matplotlib/backends/backend_macosx.py:235:
  UserWarning: Python is not installed as a framework. The MacOSX
  backend may not work correctly if Python is not installed as a
  framework. Please see the Python documentation for more information on
  installing Python as a framework on Mac OS X

I installed python27 using fink and it's using the default matplotlib is using macosx framework.