You should be able to use split() with ';' as the delimiter:

{
  ...
  "encoding": {
    "x": {
      ...
      "axis": {
        ...
        "labelExpr": "split(datum.label, ';')"
      }
    }
  },
  ...
}

I don't have your data, but I did a quick mockup with just the categories on the x channel, which hopefully satisfies your requirements: