Determines how arcs should be displayed
above_below(edgelist, above)two-column matrix
optional numeric or logical vector indicating what edges
(arcs) should be plotted above (or to the right of) of chosen axis
If above = NULL then all arcs are plotted above (or to the right)
If above is numeric, it cannot contain both positive and negative
indices.
If above is logical, its length must equal the number of rows in
edgelist
a logical vector indicating how arcs should be displayed
If horizontal = TRUE then arcs can be plotted above or
below the horizontal axis
If horizontal = FALSE then arcs can be plotted to the right or
left of the vertical axis