Vega-Lite: is it possible to plot a 3-layered graph with one Y-axis used only by 2 specific layers?

Question

I'm wondering how I can plot a single graph with multiple Y axis in a more controlled way. My current graph has already 3 layers, they're in the same values ballpark, so should remain on one Y-axis. However, now I have to plot a vastly differently-scaled thing on top, and I need an independent Y-axis just for certain layers. Is it possible?

Right now, if I set resolve: scale: Y: independent , all layers try to fight for the second Y-axis it seems, and the whole plot dissolves.

Below is a minimal reproducible example that can be copy-pasted as-is to https://vega.github.io/editor/#/ . The goal here is to be able to tell slopes of all 3 lines with a naked eye; in other words, make "XY" and "X2-Y2" lines use the left Y-axis with one scale, and make "X3-Y3" use the right Y-axis with a different one.

Please note than in reality, I already have 6 layers with different mark types and will keep adding those. All of those, however, will fall into two scale categories (say, values from 1 to 10 and from 10000 to 20000). I'd like to be able to define for each layer which category it falls into and which Y-axis - left or right - it uses.

{
  "data": {
    "values": [
      {"x": 1, "y": 10},
      {"x": 2, "y": 7},
      {"x2": 1, "y2": 11},
      {"x2": 2, "y2": 12},
      {"x3": 1, "y3": 1000},
      {"x3": 2, "y3": 2500}
    ]
  },
  "encoding": {"x": {"type": "quantitative"}, "y": {"type": "quantitative"}},
  "layer": [
    {
      "mark": "line",
      "encoding": {"x": {"field": "x"}, "y": {"field": "y"}}
      
    },
    {
      "mark": {
        "type": "square",
        "size": 100
       },
      "encoding": {
        "x": {"field": "x2"},
        "y": {"field": "y2"},
        "color": {"value": "red"}
      }
      
    },
    {
      "mark": "line",
      "encoding": {
        "x": {"field": "x3"},
        "y": {"field": "y3"},
        "color": {"value": "black"}
      }
      
    }
  ],
  "mark": "line",
  "config": {},
  "resolve": {"scale": {"y": "independent"}}
}

Answer 1

In your sample code the resolve config was given at the wrong place, and since you wanted

"XY" and "X2-Y2" lines use the left Y-axis with one scale, and make "X3-Y3" use the right Y-axis with a different one.

I have placed the 1st two layers in a separate layer which share the x and y axis and a different layer which will have independent y axis using resolve.

Check the below code or the editor link:

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "data": {
    "values": [
      {"x": 1, "y": 10},
      {"x": 2, "y": 7},
      {"x2": 1, "y2": 11},
      {"x2": 2, "y2": 12},
      {"x3": 1, "y3": 1000},
      {"x3": 2, "y3": 2500}
    ]
  },
  "encoding": {"x": {"type": "quantitative"}, "y": {"type": "quantitative"}},
  "layer": [
    {
      "layer": [
        {
          "mark": "line",
          "encoding": {"x": {"field": "x"}, "y": {"field": "y"}}
        },
        {
          "mark": "line",
          "encoding": {
            "x": {"field": "x2"},
            "y": {
              "field": "y2",
              "type": "quantitative",
              "axis": {"orient": "left"}
            },
            "color": {"value": "red"}
          }
        }
      ]
    },
    {
      "mark": "line",
      "encoding": {
        "x": {"field": "x3"},
        "y": {"field": "y3"},
        "color": {"value": "black"}
      }
    }
  ],
  "resolve": {"scale": {"y": "independent"}},
  "config": {}
}

Let me know if this was your expected outcome.

Answer 2

I came up with not the most elegant, but definitely working solution for my case. For all the layers, I'm specifying scales independently and hide axis. I feel there's more to it, and actually Wahab Memon's answer may work, if all the layers are essentially combined into only two. If I re-iterate in something more elegant, I'll post another answer.

{
    "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
    "data": {
        "values": [
            {"x": 1, "y": 10},
            {"x": 2, "y": 7},
            {"x2": 1, "y2": 11},
            {"x2": 2, "y2": 12},
            {"x3": 1, "y3": 1000},
            {"x3": 2, "y3": 2500},
            {"x4": 1, "y4": 2000},
            {"x4": 2, "y4": 4000}
        ]
    },
    "encoding": {"x": {"type": "quantitative"}, "y": {"type": "quantitative"}},
    "layer": [
        {
            "mark": "line",
            "encoding": {
                "x": {"field": "x"},
                "y": {"field": "y",
                      "type": "quantitative",
                      "axis": {"orient": "left",
                               "title": "y1, y2"
                              },
                      "scale": {"domainMin": 0,
                                "domainMax": 20}
                     },
                     "color": {"value": "blue"}
            }
        },
        {
            "mark": "line",
            "encoding": {
                "x": {"field": "x2"},
                "y": {
                    "field": "y2",
                    "type": "quantitative",
                    "axis": null,
                    "scale": {"domainMin": 0,
                              "domainMax": 20}
                },
                "color": {"value": "red"}
            }
        },

        {
            "mark": "line",
            "encoding": {
                "x": {"field": "x3"},
                "y": {"field": "y3",
                      "axis": {"orient": "right",
                               "title": "y3, y4"},
                      "type": "quantitative",
                      "scale": {"domainMin": 0,
                                "domainMax": 5000}
                     },
                "color": {"value": "black"}
            }
        },

        {
            "mark": "line",
            "encoding": {
                "x": {"field": "x4"},
                "y": {"field": "y4",
                      "axis": null,
                      "type": "quantitative",
                      "scale": {"domainMin": 0,
                                "domainMax": 5000}
                     },
                "color": {"value": "cyan"}
            }
        }

    ],
    "resolve": {"scale": {"y": "independent"}},
    "config": {}
}