This is very cool, and a great concept, i.e., to let the user see into properties of previous plots. I suppose the plan is to publish a notebook showing the mechanics of writing Plot legends, together with a few pre-packaged examples, such as tinyLegend or circleLegend, right? Isn't it bad practice, however, to add fields such as 'scales' to an SVG produced by Plot? I guess it's not such a big deal if you are doing this only for 'scales'. Another option would be to export only a 'value' field which could contain many properties besides 'scales'. I would also enjoy seeing an example with legends on the left, vega-lite style.

Remember, the 'value' of the node is already used for data flow. I don't know why it would be bad practice to add a .scales prop, but if scales is a bad name we could think of using the .__data__ property, like D3 does (but at the risk of conflicting with D3? hmmm, maybe .__plot__.scales ?).