When install ”matplotlib” with ”pip”, if you get the following error, it means the “freetype” and “png” libraries needed by matplotlib are not installed:
============================================================================
* The following required packages can not be built:
* freetype, png
============================================================================
So install them:
apt-cache search freetype | grep dev
apt-cache search libpng | grep dev
sudo apt-get install libfreetype6-dev libpng12-dev
pip install matplotlib
Done.