New anchor functions can be defined in the `joint.anchors` namespace (e.g. `joint.anchors.myAnchor`) or passed directly as a function to the `anchor` property of link source/target (or to the `defaultAnchor` option of a paper).

In either case, the anchor function must return the anchor as a Point. The function is expected to have the form `function(endView, endMagnet, anchorReference, args)`:

endView | dia.ElementView | The ElementView to which we are connecting. The Element model can be accessed as `endView.model`; this may be useful for writing conditional logic based on element attributes.
endMagnet | SVGElement | The SVGElement in our page that contains the magnet (element/subelement/port) to which we are connecting.
anchorReference | g.Point | A reference to another component of the link path that may be necessary to find this anchor point. If we are calling this method for a source anchor, it is the first vertex, or if there are no vertices the target anchor. If we are calling this method for a target anchor, it is the last vertex, or if there are no vertices the source anchor...
SVGElement | ...if the anchor in question does not exist (yet), it is that link end's magnet. (The built-in methods usually use this element's center point as reference.)
args | object | An object with additional optional arguments passed to the anchor method by the user when it was called (the `args` property).