NodeXL can handle both directed and undirected graphs.
Directed graphs are used to model asymmetric relationships. Each edge has a start point and an endpoint. In NodeXL the start point is the vertex specified in the Vertex 1 column of the Edges worksheet and the endpoint is the vertex specified in the Vertex 2 column. Arrowheads are shown at the Vertex 2 end of each edge in the Graph Pane.
Undirected graphs are used to model symmetric relationships. The edges in an undirected graph do not have start points or endpoints, and arrowheads are not shown in the Graph Pane.
Apart from determining whether arrowheads are shown, the directedness of a graph affects the way some of NodeXL's graph metrics are calculated.