包说明书：https://cran.r-project.org/web/packages/igraph/igraph.pdf

包例子：https://www.r-project.org/conferences/useR-2008/slides/Csardi.pdf

包函数：http://igraph.org/r/doc/

PPI实例：http://a-little-book-of-r-for-bioinformatics.readthedocs.io/en/latest/src/chapter11.html

其实包括了3个包：igraph/RBGL/Rgraphviz

用到了一个测试数据，是构建好的PPI网络对象：We will first analyse a curated data set of protein-protein interactions in the yeast Saccharomyces cerevisiae extracted from published papers. This data set comes from with an R package called “yeastExpData”, which calls the data set “litG”. This data was first described in a paper by Ge et al (2001) in Nature Genetics (http://www.nature.com/ng/journal/v29/n4/full/ng776.html).