graphviz简介
注意
不同于一般的Python包，graphviz需要额外下载可执行文件，并配置环境变量，如果想在python代码中使用，需要参考下面的步骤：
Install GraphViz if you haven't already (I used the MSI download)
Get the path for gvedit.exe (for me it was "C:\Program Files (x86)\Graphviz2.34\bin")
Add this path to the computer's PATH
One way to get to environment settings to set your path is to click on each of these button/menu options: start->computer->system properties->advanced settings->environment variables
Click Edit User path
Add this string to the end of your Variable value list (including semicolon): ;C:\Program Files (x86)\Graphviz2.34\bin
Click OK
Restart your Python IDE
