感谢stackoverflow,原问题网站https://stackoverflow.com/questions/35064304/runtimeerror-make-sure-the-graphviz-executables-are-on-your-systems-path-aft

解决方法:

您应该在系统中安装graphviz包(而不仅仅是python包)。在Ubuntu你应该试试:

sudo apt-get install graphviz

Note: Windows also applies