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.

El issue no menciona ningún archivo, prueba ni punto de entrada. Comienza rastreando cómo se representan las leyendas de posición y cómo se gestionan las referencias al dominio de los ejes para shapes, annotations e images; se considera terminado cuando las leyendas de posición admiten referencias arbitrarias al dominio de los ejes y admiten leyendas específicas de subplots.