see 
http://kateto.net/networks-r-igraph
[[https://rstudio-pubs-static.s3.amazonaws.com/74248_3bd99f966ed94a91b36d39d8f21e3dc3.html|igraph tutorial]]
[[http://michael.hahsler.net/SMU/LearnROnYourOwn/code/igraph.html|Examples for the igraph package]]

nodes (vertex)
edges (lines)
  * freight schedule
  * friendship
  * contact
  * relationship

social network 
{{http://s3.amazonaws.com/assets.datacamp.com/production/course_4474/datasets/networkimg.png?280}}
adjency matrix 
{{http://s3.amazonaws.com/assets.datacamp.com/production/course_4474/datasets/networkmat.png?240}}
edge list - economical way to store data
{{http://s3.amazonaws.com/assets.datacamp.com/production/course_4474/datasets/networkedg.png?85}}

<code>library(igraph)
# df = data frame data
g <- graph.edgelist(as.matrix(df), directed = FALSE)
</code>