"element" transform to complement pointer transform

[tannerlinsley]

The pointer transform is awesome, but currently we're using Plot to build some tree maps and the x/y pointer transform doesn't feel natural with "boxed" targets. I would love to see a transform that listens to hover or proximity events on the actual drawn svg nodes. This would make tooltips feel much better for plots where bounding boxes are easily hovered.

[Fil]

See also #1797.