A question about the GROUP attribute: what does STRAIGHT mean?

The web page says this about group:

If the end points of an edge belong to the same group, i.e., have the same group attribute, parameters are set to avoid crossings and keep the edges straight.

Does straight mean “edges not-multiline or splined” or does it mean “edges that are strictly horizontal or vertical”?
Or more simply, is a diagonal line (edge) straight?

When the endpoints of an edge have the same “group” attribute, the edge is assigned a large value for an attribute that penalizes edge crossings and a large "weight" attribute. The latter tries to vertically align the vertices and reduce the number of twists in the spline. Both attributes are heuristics, in that there may still be edge crossings, though fewer, and the edge can still be a non-trivial spline.