`defaultAnchor` - an anchor function that is used by links if no `anchor` property is defined for a link end. It can either be an object (with a `name` and optional `args`) or a function. JointJS provides a collection of built-in anchor functions in the `joint.anchors` namespace; alternatively, a custom function may be provided. See the Presentation section of `joint.dia.Link` for more information about anchors.