    linkView.getVertexIndex(x, y)

Return the `vertices` array index at which to insert a new vertex with the provided `x` and `y` coordinates.

The linkView finds the point on the connection that lies closest to the point `x,y`. Then, the linkView iterates over the vertices in the `link.vertices` array until one is found that lies farther than the identified closest point. The index of the farther point is returned.

The returned index can be used as the first argument of the `link.addVertex` function.