fix: properly support utc timeunits.

[haldenl]

Fixes #5777

[kanitw]

Have you test with data to avoid double shifts?

Some of the code changes here look suspicious.

[domoritz]

The docs say "Do not use UTC time unit and the UTC scale type at the same time since that will cause Vega-Lite to shift the output time twice."

But that doesn't seem to happen

{
  "data": {
    "values": [
      {"date": "Sun, 01 Jan 2012 23:00:00", "price": 150},
      {"date": "Sun, 02 Jan 2012 00:00:00", "price": 100},
      {"date": "Sun, 02 Jan 2012 01:00:00", "price": 170},
      {"date": "Sun, 02 Jan 2012 02:00:00", "price": 165},
      {"date": "Sun, 02 Jan 2012 03:00:00", "price": 200}
    ]
  },
  "mark": "line",
  "encoding": {
    "x": {
      "field": "date",
      "type": "temporal",
      "timeUnit": "utcyearmonthdatehoursminutes",
      "scale": {"type": "utc"}
    },
    "y": {"field": "price", "type": "quantitative"}
  }
}

It doesn't seem to matter whether I set the scale type to utc or not

{
  "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
  "data": {
    "values": [
      {"date": "2011-01-01T23:00:00.000+00:00"},
      {"date": "2011-01-02T00:00:00.000+00:00"},
      {"date": "2011-01-02T01:00:00.000+00:00"},
      {"date": "2011-01-02T02:00:00.000+00:00"},
      {"date": "2011-01-02T03:00:00.000+00:00"}
    ]
  },
  "mark": "line",
  "encoding": {
    "x": {
      "field": "date",
      "type": "temporal",
      "timeUnit": "utcyearmonthdatehoursminutes",
      "scale": {"type": "utc"},
      "title": null
    }
  }
}

I think that's nice but we should look into a) why this is different, b) change the docs, c) make sure that this is not a breaking change (we don't recommend using both scale type and time unit so this should be okay).

[domoritz]

Here is a sanity check I did. Looks right.

{
  "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
  "data": {
    "values": [
      {"date": "2011-01-01T23:00:00.000+00:00"},
      {"date": "2011-01-02T00:00:00.000+00:00"},
      {"date": "2011-01-02T01:00:00.000+00:00"},
      {"date": "2011-01-02T02:00:00.000+00:00"},
      {"date": "2011-01-02T03:00:00.000+00:00"}
    ]
  },
  "mark": "point",
  "encoding": {
    "y": {
      "field": "date",
      "type": "ordinal",
      "timeUnit": "utcyearmonthdatehoursminutes",
      "title": null
    }
  }
}

[domoritz]

Another experiment

{
  "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
  "data": {
    "values": [
      {"t": "2011-01-02T00:00:00.000+00:00"},
      {"t": "2011-01-02T01:00:00.000+00:00"},
      {"t": "2011-01-02T02:00:00.000+00:00"},
      {"t": "2011-01-02T03:00:00.000+00:00"}
    ]
  },
  "mark": "point",
  "encoding": {
    "x": {
      "type": "temporal",
      "field": "t",
      "timeUnit": "hours",
      "title": "hours"
    },
    "y": {
      "type": "temporal",
      "field": "t",
      "timeUnit": "utchours",
      "title": "utchours"
    }
  }
}

[haldenl]

ready to merge

[kanitw]

I exclude spec changes that are irrelevant to time unit.

I also made a bit updates to the examples to make it even easier to read.

Feel free to merge after all tests pass.

[haldenl]

Thanks Ham!

[jakevdp]

Thanks for your work on this!