这两个都是因为用到snap的时候要画图和显示图的库。
windows下的安装还是挺容易的。gnuplot是有64位的库，graphviz是32位的。装完后，都记得要把路径改下，这样子才能成功的使用。
snap中有段测试代码分别测试gnuplot和graphviz，gnuplot的挺easy的。graphviz的其实也挺容易的，只是运行完之后，不是立马出来了结果，会产生一个dot文件。然后打开graphviz查看dot文件。虽然dot文件也可以用word打开。是一段转换后的代码。
具体可以参见这个解释在stackoverflow上面。http://stackoverflow.com/questions/1494492/graphviz-how-to-go-from-dot-to-a-graph、
原文如下：
type: dot -Tps filename.dot -o outfile.ps
If you want to use the dot renderer. There are alternatives like neato and twopi. If graphiz isn‘t in your path, figure out where it is installed and run it from there.
If you‘re using windows, check out the installed tool called GVEdit, it makes the whole process slightly easier.
Go look at the graphviz site in the section called "User‘s Guides" for more detail on how to use the tools:
http://www.graphviz.org/Documentation.php
(See page 27 for output formatting for the dot command, for instance)
http://www.graphviz.org/pdf/dotguide.pdf