Deploy matplotlib on heroku failed. How to do this correctly?

Deploy matplotlib on heroku failed. How to do this correctly?

I've install matplotlib in my virtualenv using pip. It was a failure at
the beginning, but after I do easy_install -U distribute, the installation
goes smoothly.
Here is what I do (inside my git repository root folder):
virtualenv env
source env/bin/activate
pip install gunicorn
pip install numpy
easy_install -U distribute
pip install matplotlib
Then, I make a requirements.txt by using pip freeze > requirements.txt.
Here is the result:
argparse==1.2.1
distribute==0.7.3
gunicorn==17.5
matplotlib==1.3.0
nose==1.3.0
numpy==1.7.1
pyparsing==2.0.1
python-dateutil==2.1
six==1.3.0
tornado==3.1
wsgiref==0.1.2
Problem happened when I try to deploy my application:
(env)gofrendi@kirinThor:~/kokoropy$ git push -u heroku
Counting objects: 9, done.
Delta compression using up to 2 threads.
Compressing objects: 100% (5/5), done.
Writing objects: 100% (5/5), 586 bytes, done.
Total 5 (delta 3), reused 0 (delta 0)
-----> Python app detected
-----> No runtime.txt provided; assuming python-2.7.4.
-----> Using Python runtime (python-2.7.4)
-----> Installing dependencies using Pip (1.3.1)
Downloading/unpacking distribute==0.7.3 (from -r requirements.txt
(line 2))
Running setup.py egg_info for package distribute
Downloading/unpacking matplotlib==1.3.0 (from -r requirements.txt
(line 4))
Running setup.py egg_info for package matplotlib
The required version of distribute (>=0.6.28) is not available,
and can't be installed while this script is running. Please
install a more recent version first, using
'easy_install -U distribute'.
(Currently using distribute 0.6.24
(/app/.heroku/python/lib/python2.7/site-packages))
Complete output from command python setup.py egg_info:
The required version of distribute (>=0.6.28) is not available,
and can't be installed while this script is running. Please
install a more recent version first, using
'easy_install -U distribute'.
(Currently using distribute 0.6.24
(/app/.heroku/python/lib/python2.7/site-packages))
----------------------------------------
Command python setup.py egg_info failed with error code 2 in
/tmp/pip-build-u55833/matplotlib
Storing complete log in /app/.pip/pip.log
! Push rejected, failed to compile Python app
To git@heroku.com:kokoropy.git
! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'git@heroku.com:kokoropy.git'
(env)gofrendi@kirinThor:~/kokoropy$
Seems that heroku server can't install matplotlib correctly.
When I do easy_install -U distribute it might not being recorded by pip.
Matplotlib also has several non-python-library dependencies (such as:
libjpeg8-dev, libfreetype and libpng6-dev). I can install those
dependencies locally (e.g: via apt-get). However, this also not being
recorded by pip.
So, my question is: how to correctly install matplotlib in heroku
deployment server?