Deneb - 3x3 subplot

Hi patBear,

This should be achievable with a stack transform at the top-level, as you have guessed. To make the stack work better, I've added a window / row_number transform beforehand to group and sort each square by z:

  "transform": [
    {
      "window": [
        {
          "op": "row_number",
          "as": "group_row_number"
        }
      ],
      "groupby": ["x", "y"],
      "sort": [{"field": "z"}]
    },
    {
      "stack": "group_row_number",
      "groupby": ["x", "y"],
      "sort": [{"field": "z"}],
      "as": ["y1", "y2"]
    }
  ]

This has the effect of assigning a sequence to each row as follows:

We then use the same group approach in the stack, and use the group_row_number as the field to stack on.

Once this is done, I just need to use a yOffset in the encoding with the generated y1 field:

    "yOffset": {
      "field": "y1",
      "scale": {"reverse": true}
    }

One thing I've done is reverse the scale so that marks are drawn from the bottom of each square.

The resulting visual looks as follows:

For completeness, here's the full spec:

{
  "data": {"name": "dataset"},
  "transform": [
    {
      "window": [
        {
          "op": "row_number",
          "as": "group_row_number"
        }
      ],
      "groupby": ["x", "y"],
      "sort": [{"field": "z"}]
    },
    {
      "stack": "group_row_number",
      "groupby": ["x", "y"],
      "sort": [{"field": "z"}],
      "as": ["y1", "y2"]
    }
  ],
  "layer": [
    {
      "mark": {
        "type": "bar",
        "stroke": "black",
        "strokeWidth": 1,
        "tooltip": true
      }
    },
    {
      "mark": "text",
      "encoding": {
        "text": {
          "field": "Name",
          "type": "nominal"
        }
      }
    }
  ],
  "encoding": {
    "x": {
      "field": "x",
      "type": "nominal",
      "scale": {
        "domain": ["low", "med", "high"]
      },
      "axis": {
        "title": "X Level",
        "labelAngle": 0
      }
    },
    "y": {
      "field": "y",
      "type": "nominal",
      "scale": {
        "domain": ["high", "med", "low"]
      },
      "axis": {"title": "Y Level"}
    },
    "yOffset": {
      "field": "y1",
      "scale": {"reverse": true}
    }
  }
}

Hopefully this is what you're expecting, but should be possible to tweak to your liking if not.

Good luck (and thanks for considering Deneb)!

Daniel