Improve docs for dots in signal name

Hi, firstly thanks for the library =)… It’s great and really usable, I did however have a surprising interaction which I’ll detail below.

I rendered a longer version of the vega-lite spec below and it generated no output, I pulled up the warnings and saw No error or warnings. I only realised the problem after digging into this issue on the vega issues list.

It would be helpful if any labels are not valid/may not be valid would throw up a warning? As that’s not something I would expect to normally be a problem…

I doubt I personally will trip over this in the future, but it would be a nice addition to the already existing suit of feedback you get when you mess up.

{
  "data": {
    "values": [
      {
        "day": 1,
        "measurement": -45.681350765351375,
        "estimate-0.1": -0.06813507653513806,
        "estimate-0.4": -15.272540306140552,
        "estimate-0.8": -35.545080612281104
      },
      {
        "day": 2,
        "measurement": 47.13935530761721,
        "estimate-0.1": 3.7403496481037726,
        "estimate-0.4": 9.084041730178338,
        "estimate-0.8": 30.399742720576143
      },
      {
        "day": 3,
        "measurement": -12.085906115629815,
        "estimate-0.1": 2.1134398711443576,
        "estimate-0.4": 0.7689926542196375,
        "estimate-0.8": -3.4567094997092127
      }
    ]
  },
  "transform": [
    {"fold": ["measurement", "estimate-0.1", "estimate-0.4", "estimate-0.8"]}
  ],
  "width": 800,
  "height": 600,
  "mark": {"type": "line", "tooltip": {"content": "data"}, "point": true},
  "encoding": {
    "x": {"field": "day", "type": "quantitative"},
    "y": {
      "field": "value",
      "type": "quantitative",
      "scale": {"domain": [-45, 310]}
    },
    "color": {"field": "key", "type": "nominal"}
  }
}

this clearly does:

{
  "data": {
    "values": [
      {
        "day": 1,
        "measurement": -45.681350765351375,
        "estimate-01": -0.06813507653513806,
        "estimate-04": -15.272540306140552,
        "estimate-08": -35.545080612281104
      },
      {
        "day": 2,
        "measurement": 47.13935530761721,
        "estimate-01": 3.7403496481037726,
        "estimate-04": 9.084041730178338,
        "estimate-08": 30.399742720576143
      },
      {
        "day": 3,
        "measurement": -12.085906115629815,
        "estimate-01": 2.1134398711443576,
        "estimate-04": 0.7689926542196375,
        "estimate-08": -3.4567094997092127
      }
    ]
  },
  "transform": [
    {"fold": ["measurement", "estimate-01", "estimate-04", "estimate-08"]}
  ],
  "width": 800,
  "height": 600,
  "mark": {"type": "line", "tooltip": {"content": "data"}, "point": true},
  "encoding": {
    "x": {"field": "day", "type": "quantitative"},
    "y": {
      "field": "value",
      "type": "quantitative",
      "scale": {"domain": [-45, 310]}
    },
    "color": {"field": "key", "type": "nominal"}
  }
}

[domoritz]

I’ll close it when I think the docs are good enough.

[domoritz]

Thank you so much for raising the issue and describing an improvement. It’s really helpful to hear this kind of feedback.

I don’t think I want to add a default warning whenever a field name uses . but we should definitely make sure that the docs are clear. In the future, we should improve tracking of what fields are used, which would enable warnings for any kind of unrecognized field.

The tracking feature request is in https://github.com/vega/vega-lite/issues/2862. I am marking this as a documentation request. Sounds good?