Add axis domain as an option for position legends

This is related to this question in the community forum. Opened an issue since I think this is a better place for a feature request.

In short, enabling arbitrary axis as a reference system for legends would greatly simplify its positioning along different axis in order to achieve a legend per plot in a figure with subplots. Currently is only supported for shapes, annotations, and images:

Furthermore, shapes, annotations, and images can be placed relative to an axis's domain so that, for instance, an x value of 0.5 would place the object halfway along the x-axis, regardless of the domain as specified in the layout.xaxis.domain attribute. This behavior can be specified by adding ' domain' to the axis reference in the axis referencing attribute of the object. For example, setting yref = 'y2 domain' for a shape will refer to the length and position of the axis named y2.

The issue names no file, test, or entry point. Begin by tracing how position legends are represented and how axis-domain references are handled for shapes, annotations, and images; done means arbitrary axis-domain references work for position legends and support subplot-specific legends.