Facet of concat does not work

[jakevdp]

Faceting of concatenated charts appears to be supported by the renderer, but not allowed by the schema. This means that tools like Altair, which require schema-validated chart specifications, cannot create this type of chart.

Here is an example (vega editor):

{
  "spec": {
    "vconcat": [
      {
        "mark": "point",
        "encoding": {
          "x": {"field": "Horsepower", "type": "quantitative"},
          "y": {"field": "Miles_per_Gallon", "type": "quantitative"}
        }
      },
      {
        "mark": "tick",
        "encoding": {
          "x": {"field": "Horsepower", "type": "quantitative"}
        }
      }
    ]
  },
  "data": {"url": "data/cars.json"},
  "facet": {"field": "Origin", "type": "nominal"}
}

It renders as expected, however the specification does not meet the schema requirements:

# python 3
from urllib.request import urlopen
import json
import jsonschema

schema = json.load(urlopen("https://raw.githubusercontent.com/vega/schema/master/vega-lite/v3.4.0.json"))
spec = json.loads("""
{
  "spec": {
    "vconcat": [
      {
        "mark": "point",
        "encoding": {
          "x": {"field": "Horsepower", "type": "quantitative"},
          "y": {"field": "Miles_per_Gallon", "type": "quantitative"}
        }
      },
      {
        "mark": "tick",
        "encoding": {
          "x": {"field": "Horsepower", "type": "quantitative"}
        }
      }
    ]
  },
  "data": {"url": "data/cars.json"},
  "facet": {"field": "Origin", "type": "nominal"}
}
""")
jsonschema.validate(spec, schema)

---------------------------------------------------------------------------
ValidationError                           Traceback (most recent call last)
<ipython-input-6-df00bc6f0e25> in <module>()
     27 }
     28 """)
---> 29 jsonschema.validate(spec, schema)

1 frames
/usr/local/lib/python3.6/dist-packages/jsonschema/validators.py in validate(self, *args, **kwargs)
    128         def validate(self, *args, **kwargs):
    129             for error in self.iter_errors(*args, **kwargs):
--> 130                 raise error
    131 
    132         def is_type(self, instance, type):

ValidationError: {'spec': {'vconcat': [{'mark': 'point', 'encoding': {'x': {'field': 'Horsepower', 'type': 'quantitative'}, 'y': {'field': 'Miles_per_Gallon', 'type': 'quantitative'}}}, {'mark': 'tick', 'encoding': {'x': {'field': 'Horsepower', 'type': 'quantitative'}}}]}, 'data': {'url': 'data/cars.json'}, 'facet': {'field': 'Origin', 'type': 'nominal'}} is not valid under any of the given schemas

Failed validating 'anyOf' in schema:
    {'anyOf': [{'$ref': '#/definitions/TopLevelUnitSpec'},
               {'$ref': '#/definitions/TopLevelFacetSpec'},
               {'$ref': '#/definitions/TopLevelLayerSpec'},
               {'$ref': '#/definitions/TopLevelRepeatSpec'},
               {'$ref': '#/definitions/TopLevelConcatSpec'},
               {'$ref': '#/definitions/TopLevelVConcatSpec'},
               {'$ref': '#/definitions/TopLevelHConcatSpec'}],
     'description': 'A Vega-Lite top-level specification.\n'
                    'This is the root class for all Vega-Lite '
                    'specifications.\n'
                    '(The json schema is generated from this type.)'}

[domoritz]

We only support unit and layer inside facet right now since there are some complicated layout issues to solve otherwise. We are tracking this issue in #2760.

For now, you are supposed to swap concat and facet but somehow this breaks:

{
  "$schema": "https://vega.github.io/schema/vega-lite/v3.json",
  "data": {"url": "data/cars.json"},
  "vconcat": [
    {
      "facet": {"field": "Origin", "type": "nominal"},
      "spec": {
        "mark": "point",
        "encoding": {
          "x": {"field": "Horsepower", "type": "quantitative"},
          "y": {"field": "Miles_per_Gallon", "type": "quantitative"}
        }
      }
    },
    {
      "facet": {"field": "Origin", "type": "nominal"},
      "spec": {
        "mark": "tick",
        "encoding": {
          "x": {"field": "Horsepower", "type": "quantitative"}
        }
      }
    }
  ]
}

We can tackle these issues (and in particular #2760) after refactoring repeat: #4947.

[joelostblom]

Just a note here that concat of facet works since #7639 so updating the title of this issue.

[domoritz]

Thank you. Facet of concat is a lot tricker to get right and less important imo. I'm glad we got concat of X working.