Error: libpng15.15.dylib & _png.so in matplotlib (pyplot)

原因は不明です。OnyXで何か消したのか、homebrewのアップデートで何かが変わったのか、、、。
とりあえず以下のように対処して解決しました。

Error:
ImportError: dlopen(/usr/local/lib/python2.7/site-packages/matplotlib/_png.so, 2): Library not loaded: /usr/local/lib/libpng15.15.dylib
Referenced from: /usr/local/lib/python2.7/site-packages/matplotlib/_png.so
Reason: image not found

対処:
sudo pip uninstall matplotlib
sudo rm -rf /tmp/pip-build-root/
brew reinstall libpng --universal
brew reinstall freetype --universal
sudo ln -s /usr/local/opt/freetype/include/freetype2 /usr/local/include/freetype
sudo pip install matplotlib

Thanks to the following:
http://d.hatena.ne.jp/mutsune/20140416/1397617050
http://stackoverflow.com/questions/20572366/sudo-pip-install-matplotlib-fails-to-find-freetype-headers-os-x-mavericks

追記：libpng warning: Application built with libpng-1.5.17 but running with 1.6.10 というエラーに見舞われた。
対処

sudp rm /opt/X11/include/libpng15/png*.h

launchctl unload /Library/LaunchAgents/org.macosforge.xquartz.startx.plist
sudo launchctl unload /Library/LaunchDaemons/org.macosforge.xquartz.privileged_startx.plist
sudo rm -rf /opt/X11* /Library/Launch*/org.macosforge.xquartz.* /Applications/Utilities/XQuartz.app /etc/*paths.d/*XQuartz
sudo pkgutil --forget org.macosforge.xquartz.pkg

で再度matplotlibをuninstall install.
Thanks to
http://stackoverflow.com/questions/22898245/libpng-version-incompatibility-in-fresh-installation-of-ipython
http://xquartz.macosforge.org/trac/wiki/X11-UsersFAQ#UninstallSnowLeopardorLater