subplot duplication in overlayed axis #2347

Scenario: I have a plot with two y axis overlayed where trace data is loaded asynchronously so there is no guarantee of order. When a trace that belongs to the y axis overlaying the main y axis is loaded first, the plot generate 2 set o graphic elements on SVG representing the trace: the first as a subplot and the second as an overplot of the mainplot.

Here is a live example: https://codepen.io/vinipitta/pen/WMGdbx
(Just drag the defective trace (trace 1) to see the duplicated trace remain still while a copy moves on the right y axis.)

Looks like since there is no trace loaded in the main y axis, the plotinfo.mainplot is undefined when the trace is added to the overlayed axis generating a subplot. When a trace is added to the mainplot, the overplot group is populated with the previous trace and the former subplot ins't removed.

function makeSubplotLayer(plotinfo) {
    var plotgroup = plotinfo.plotgroup;
    var id = plotinfo.id;
    var xLayer = constants.layerValue2layerClass[plotinfo.xaxis.layer];
    var yLayer = constants.layerValue2layerClass[plotinfo.yaxis.layer];

    if(!plotinfo.mainplot) {

The simplest workaround is to set the layout.yaxis.overlaying or layout.yaxis2.overlaying based on where the first loaded trace belongs to.

This issue seems related to: #2227