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.