Graphviz's executables are not found (Python 3.4)
In my case (Win10, Anaconda3, Jupyter notebook) after "conda install graphviz" I have to add to the PATH: C:\Users\username\Anaconda3\Library\bin\graphviz
To modify PATH goto Control Panel > System and Security > System > Advanced System Settings > Environment Variables > Path > Edit > New
Because Mac OS has not been mentioned I will add that I had the same problem on OS X Yosemite, the solution I found was to do brew install graphviz
This solved the problem, not sure if I shouldn't have just edited one of the other answers in this list, because they all seem to be the same answer, just install an official package in addition to the Python Library.