There is currently no stable (working) version of matplotlib out there for python 3.6. But there is a release candidat with version number: 2.0.0rc2. You are able to install this version without any problem. Just specify this version on install:
pip install matplotlib==2.0.0rc2
If you have installed any other Version or if you want to upgrade lateron use the
-I switch of pip like:
pip install -I matplotlib==2.0.0rc2
... to ignore the installed package and reinstall it instead.
If you try this later, check the newest version and change it, to avoid an error.
Later on, if matplotlib is stable, the error sould be gone. But for now, this is the only working solution. The Problem in this case is not matplotlib itself, it is caused by the packages "png" and "Freetype".