A link anchor of a link is a point on the reference link that this link wants to reach as its endpoint. Link anchors are set via a `linkAnchor` property provided within link end definitions (i.e. the objects provided to `link.source()` and `link.target()` functions). (If the reference object is an Element, JointJS looks at `anchor` property instead.)

There are several built-in linkAnchor functions in JointJS:

- `'connectionRatio'` - default link anchor at specified ratio at reference link
- `'connectionLength'` - link anchor at specified length along reference link
- `'connectionPerpendicular'` - link anchor that ensures an orthogonal route to the reference link
- `'connectionClosest'` - link anchor at the point on reference link that is closest to the last vertex on link path

Example:

    link.source(link2, {
        linkAnchor: {
            name: 'connectionRatio',
            args: {
                ratio: 0.25
            }
        }
    });

The default link anchor function is `'connectionRatio'`; this can be changed with the `defaultLinkAnchor` paper option. Example:

    paper.options.defaultLinkAnchor = {
        name: 'connectionLength',
        args: {
            length: 20
        }
    };

JointJS also contains mechanisms to define one's own custom link anchor functions.