basically a two-column matrix with edges
(see graph)
vertices
optional vector of vertex names
corresponding with those in the edgelist
sorted
logical to indicate if nodes should be
sorted (default FALSE)
decreasing
logical to indicate type of sorting
(used only when sorted=TRUE)
ordering
optional numeric or string vector
providing the ordering of nodes. When provided, this
parameter overrides sorted=TRUE). See the details
section for more information.