Xaxis and yaxis indices for subplots with multiple axes

plotly.js
1

I have just started with Plotly. I am confused about the numbering convention for xaxis and yaxis in subplots. The following code makes two subplots, each with two shared x-axis plot:

var data = [
  { xaxis: "x", yaxis: "y", x: [1, 2, 3], y: [10, 20, 30], type: "scatter" },
  { xaxis: "x", yaxis: "y3", x: [2, 3, 4], y: [7, 6, 5], type: "scatter" },
  { xaxis: "x2", yaxis: "y2", x: [3, 4, 5], y: [40, 50, 60], type: "scatter" },
  { xaxis: "x2", yaxis: "y4", x: [4, 5, 6], y: [4, 5, 6], type: "scatter" },
];

var layout = {
  grid: { pattern: "independent", rows: 1, columns: 2 },
  yaxis3: { overlaying: "y", anchor: "x", side: "right" },
  yaxis4: { overlaying: "y2", anchor: "x2", side: "right" }
};

Plotly.newPlot("myDiv", data, layout);

see https://codepen.io/mzaffalon/pen/wvdBRPZ

What does not seem to work is when the main plot has the index for xaxis and yaxis different, that is when I replace the expression above xaxis: "x2", yaxis: "y2" by xaxis: "x2", yaxis: "y3" as in the code below:

var data = [
  { xaxis: "x", yaxis: "y", x: [1, 2, 3], y: [10, 20, 30], type: "scatter" },
  { xaxis: "x", yaxis: "y2", x: [2, 3, 4], y: [7, 6, 5], type: "scatter" },
  { xaxis: "x2", yaxis: "y3", x: [3, 4, 5], y: [40, 50, 60], type: "scatter" },
  { xaxis: "x2", yaxis: "y4", x: [4, 5, 6], y: [4, 5, 6], type: "scatter" },
];

var layout = {
  grid: { pattern: "independent", rows: 1, columns: 2 },
  yaxis2: { overlaying: "y", anchor: "x", side: "right" },
  yaxis4: { overlaying: "y3", anchor: "x2", side: "right" }
};

Plotly.newPlot("myDiv", data, layout);

see https://codepen.io/mzaffalon/pen/zYwxyyd

I could not find any documentation about the numbering convention: is my observation correct?