Nested axis labels for grouped bar chart in Vega Lite

I'm building a grouped bar chart using Vega-Lite. Every data point has a count, a sample and an fdr value. The bars are grouped per sample and in these groups, I want one bar per fdr-value. I was able to accomplish all of this using the following specification (click here for a version in the Vega editor):

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "data": {
    "values": [
      {"count": 87, "organism": "Thaliana", "sample": "S01", "fdr": "0.1%"},
      {"count": 231, "organism": "Spider", "sample": "S01", "fdr": "0.1%"},
      {"count": 40, "organism": "Thaliana", "sample": "S01", "fdr": "1%"},
      {"count": 560, "organism": "Spider", "sample": "S01", "fdr": "1%"},
      {"count": 11.33, "organism": "Thaliana", "sample": "S02", "fdr": "0.1%"},
      {"count": 88.67, "organism": "Spider", "sample": "S02", "fdr": "0.1%"},
      {"count": 70, "organism": "Thaliana", "sample": "S02", "fdr": "1%"},
      {"count": 30, "organism": "Spider", "sample": "S02", "fdr": "1%"}
    ]
  },
  "width": "container",
  "layer": [
    {
      "mark": "bar",
      "encoding": {
        "x": {
          "aggregate": "sum",
          "field": "count",
          "stack": "normalize",
          "title": "Distribution (%)",
          "sort": "descending",
          "axis": {"labelFontSize": 14, "titleFontSize": 20}
        },
        "y": {
          "field": "sample",
          "title": "Sample name",
          "axis": {
            "labelFontSize": 14,
            "titleFontSize": 20,
            "labelLimit": 500,
            "labelExpr": "datum.label"
          }
        },
        "yOffset": {"field": "fdr", "scale": {"paddingInner": 0.1}},
        "color": {"field": "organism", "legend": {"title": "Species"}}
      }
    }
  ],
  "config": {
    "legend": {
      "orient": "bottom",
      "direction": "vertical",
      "columns": 4,
      "labelFontSize": 14,
      "titleFontSize": 16,
      "labelLimit": 250,
      "symbolSize": 150
    }
  }
}

A screenshot of the current visualization:

Now, the issue that I have is that I also want to add a label showing the fdr-value per bar (since every bar only corresponds to one fdr). But I cannot manage to get this working because the yOffset channel doesn't support its own axis.

Any ideas on how I can accomplish this?

EDIT: This is an example of approximately what it is that I want to achieve:

Do you mean like this?

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "data": {
    "values": [
      {"count": 87, "organism": "Thaliana", "sample": "S01", "fdr": "0.1%"},
      {"count": 231, "organism": "Spider", "sample": "S01", "fdr": "0.1%"},
      {"count": 40, "organism": "Thaliana", "sample": "S01", "fdr": "1%"},
      {"count": 560, "organism": "Spider", "sample": "S01", "fdr": "1%"},
      {"count": 11.33, "organism": "Thaliana", "sample": "S02", "fdr": "0.1%"},
      {"count": 88.67, "organism": "Spider", "sample": "S02", "fdr": "0.1%"},
      {"count": 70, "organism": "Thaliana", "sample": "S02", "fdr": "1%"},
      {"count": 30, "organism": "Spider", "sample": "S02", "fdr": "1%"}
    ]
  },
  "width": "container",
  "encoding": {
    "y": {
      "field": "sample",
      "title": "Sample name",
      "axis": {
        "labelFontSize": 14,
        "titleFontSize": 20,
        "labelLimit": 500,
        "labelExpr": "datum.label"
      }
    },
    "yOffset": {"field": "fdr", "scale": {"paddingInner": 0.1}}
  },
  "layer": [
    {
      "mark": "bar",
      "encoding": {
        "x": {
          "aggregate": "sum",
          "field": "count",
          "stack": "normalize",
          "title": "Distribution (%)",
          "sort": "descending",
          "axis": {"labelFontSize": 14, "titleFontSize": 20}
        },
        "color": {"field": "organism", "legend": {"title": "Species"}}
      }
    },
    {
      "mark": {"type": "text", "opacity": 0.9},
      "encoding": {
        "x": {
          "aggregate": "sum",
          "field": "count",
          "sort": "descending",
          "stack": "normalize",
          "bandPosition": 0.5
        },
        "text": {"field": "fdr"},
        "detail": {"field": "organism"},
        "color": {"value": "white"}
      }
    }
  ],
  "config": {
    "legend": {
      "orient": "bottom",
      "direction": "vertical",
      "columns": 4,
      "labelFontSize": 14,
      "titleFontSize": 16,
      "labelLimit": 250,
      "symbolSize": 150
    }
  }
}

Edit:

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "data": {
    "values": [
      {"count": 87, "organism": "Thaliana", "sample": "S01", "fdr": "0.1%"},
      {"count": 231, "organism": "Spider", "sample": "S01", "fdr": "0.1%"},
      {"count": 40, "organism": "Thaliana", "sample": "S01", "fdr": "1%"},
      {"count": 560, "organism": "Spider", "sample": "S01", "fdr": "1%"},
      {"count": 11.33, "organism": "Thaliana", "sample": "S02", "fdr": "0.1%"},
      {"count": 88.67, "organism": "Spider", "sample": "S02", "fdr": "0.1%"},
      {"count": 70, "organism": "Thaliana", "sample": "S02", "fdr": "1%"},
      {"count": 30, "organism": "Spider", "sample": "S02", "fdr": "1%"}
    ]
  },
  "width": 500,
  "mark": "bar",
  "encoding": {
   
   "row": {"field": "sample", "header":{"labelFontSize":20, "labelFontWeight":"bold", "labelAngle": 0, "labelPadding":-30, "title":"Sample Name", "titleFontSize":20}},
    "y": {
      "field": "fdr",
       "axis": {"labelFontSize": 12, "titleFontSize": 0, "labelExpr": "'FDR ' + datum.value", "labelAlign":"left", "labelPadding":60}
    },
   
    "x": {
      "aggregate": "sum",
      "type": "quantitative",
      "field": "count",
      "stack": "normalize",
      "title": "Distribution (%)",
      "sort": "descending",
      "axis": {"labelFontSize": 14, "titleFontSize": 20}
       },
    "color": {"field": "organism", "legend": {"title": "Species"}}
  },
  "config": {
    "legend": {
      "orient": "bottom",
      "direction": "vertical",
      "columns": 4,
      "labelFontSize": 14,
      "titleFontSize": 16,
      "symbolSize": 150
    }
  }
}