Feature request: access to layers and subplots with stable API

Context

We want to display interactive SVG elements on a plotly plot in data coordinates. In order to do this, we currently use the _fullLayout "private" API in order to get a handle to

• subplots
• clipId
• top shape layer.

We then add a new layer for positioning interactive elements right above _fullLayout._shapeUpperLayer, as seen here:

import { select as d3_select } from 'd3-selection';

export function getInteractorLayer(plotlyPlot, plot='xy') {
  // plotlyPlot is a plotly plot object, e.g. from Plotly.newPlot() or Plotly.react()
  // plot is the subplot name, e.g. 'xy', 'x2y2'
  const subplot = plotlyPlot._fullLayout._plots[plot];
  const shapelayer = plotlyPlot._fullLayout._shapeUpperLayer;
  const layer_above = d3_select(shapelayer.node().parentNode);
  // create interactor layer, if not exists:
  layer_above.selectAll("g.interactorlayer")
    .data(["interactors"])
    .enter().append("g")
    .classed("interactorlayer", true);
  const interactorlayer = layer_above.selectAll("g.interactorlayer");
  const clipid = subplot.clipId.replace(/plot$/, '');
  return { interactorlayer, subplot, clipid };
}

Request

A public interface for getting handles to subplots, the clipId, and the top shape layer (or a simple API function for creating a new top layer!)

Once we have the items above, we can create interactors on top of a plotly plot and re-render them as needed with e.g.

  plotlyPlot.on('plotly_relayout', relayout);
  plotlyPlot.on('plotly_afterplot', relayout);
  plotlyPlot.on('plotly_relayouting', relayout);

as seen in this demo: https://bmaranville.github.io/plotly-interactors/demo.html (drag the interactors around and look at the console to see the callbacks being called with the updated state of the interactor)

Start with the getInteractorLayer example and the plotly_relayout, plotly_afterplot, and plotly_relayouting entry points described in the issue. Define a public alternative to the private _fullLayout access that exposes subplots, clipId, and the top shape layer or creates a top layer, then verify that interactors can be positioned and re-rendered through the demonstrated callbacks.