The `'center'` anchor function is the default anchor function. It places the anchor of the link at center of the view bbox. It accepts three arguments, which can be passed within the `anchor.args` property:

rotate | boolean | Should the anchor bbox rotate with the end view? Default is `false`, meaning that the unrotated bbox is used.
dx | number | Offset the anchor by `dx`. Default is `0`.
string | Percentage strings (e.g. `'40%'`) are also accepted.
dy | number | Offset the anchor by `dy`. Default is `0`.
string | Percentage strings (e.g. `'40%'`) are also accepted.

Example:

    link.source(model, {
        anchor: {
            name: 'center',
            args: {
                rotate: true,
                dx: 10,
                dy: '40%'
            }
        }
    });