Add a new vertex (an object with `x` and `y` properties) to the link. Normally, vertices are set through the `vertices` property of link models. However, sometimes it is useful to add a new vertex to the link and let the link determine to what index such a vertex should be added in the `vertices` array. For that, it must be the link view that does that. The link view tries to add the vertex to different indices in the `vertices` array, renders the connection SVG path and checks if the path length changed significantly. If it did, it tries another index. If not, the found index is most likely to be the one we want our new vertext to be added to. For instance, let's say you want to add a new vertex on double click instead of the default click event. You can create your paper in the following way:

    var paper = new joint.dia.Paper({
        // ...
        linkView: joint.dia.LinkView.extend({
            pointerdblclick: function(evt, x, y) {
                if (V(evt.target).hasClass('connection') || V(evt.target).hasClass('connection-wrap')) {
                    this.addVertex({ x: x, y: y });
                }
            }
        }),
        interactive: function(cellView) {
            if (cellView.model instanceof joint.dia.Link) {
                // Disable the default vertex add functionality on pointerdown.
                return { vertexAdd: false };
            }
            return true;
        }
        // ...
    });