A connection or a string giving the file name to write the graph
to.
format
Character string giving the file format. Right now
pajek, graphml, dot, gml, edgelist,
lgl, ncol and dimacs are implemented. As of igraph 0.4
this argument is case insensitive.
...
Other, format specific arguments, see below.
Edge list format
The edgelist format is a simple text file,
with one edge in a line, the two vertex ids separated by a space character.
The file is sorted by the first and the second column. This format has no
additional arguments.
Adai AT, Date SV, Wieland S, Marcotte EM. LGL: creating a map of
protein function with an algorithm for visualizing very large biological
networks. J Mol Biol. 2004 Jun 25;340(1):179-90.