Designate another subelement of the cell as a proxy target for the cell's magnet, whenever this subelement (the one on which this attribute is defined) becomes the source/target of a link. (Usually, this attribute needs to be defined on the `'root'` subelement of the cell, since that is the default target for the cell's magnet.) Expects a selector.

A change in the cell's magnet affects the following characteristics of the cell view:

- link anchor
- link connection point
- connector
- router

It has no effect on the connection validation, magnet highlighter and the `source` and `target` link model attributes.

    // `standard.Rectangle` has `root`, `body` and `label` subelements
    const rect = new joint.shapes.standard.Rectangle();
    // we want to have the `label` on the outside and under the `body` of the rectangle:
    rect.attr(['label'], { refY: '100%', textVerticalAnchor: 'top' });
    // normally, links would connect to the `root` subelement (the wrapper of `body` and `label`)
    // however, we want links to connect only to the `body` subelement, so we need to specify it as a proxy for `root`:
    rect.attr(['root','magnetSelector'], 'body');