name of the end node. For symmetric edges (bidirected and undirected), the
order of start and end node is arbitrary.
e
type of edge. Can be one of "->", "<->" and "--".
x
X coordinate for a control point. If this is not NA, then the edge
is drawn as an xspline through the start point, this control point,
and the end point. This is especially important for cases where there is more than
one edge between two variables (for instance, both a directed and a bidirected edge).