Binning shows a "null" label when there is only 1 bin

[chris-canipe]

Binning shows a "null" label when there is only one bin.

Vega Editor example

{
  "$schema": "https://vega.github.io/schema/vega-lite/v3.0.0-rc12.json",
  "mark": "bar",
  "encoding": {
    "x": {"aggregate": "count", "field": "*", "type": "quantitative"},
    "y": {
      "bin": true,
      "field": "value",
      "type": "nominal"
    }
  },
  "data": {
    "values": [
      {
        "value": "1"
      }
    ]
  }
}

It works properly if additional data is added that will result in 2 or more bins:

{
  "$schema": "https://vega.github.io/schema/vega-lite/v3.0.0-rc12.json",
  "mark": "bar",
  "encoding": {
    "x": {"aggregate": "count", "field": "*", "type": "quantitative"},
    "y": {
      "bin": true,
      "field": "value",
      "type": "nominal"
    }
  },
  "data": {
    "values": [
      {
        "value": "1"
      },
      {
        "value": "2"
      }
    ]
  }
}

[kanitw]

The problem is caused by the extent to the bin transform.

A workaround is to change the extent signal to account for one value case:

if(bin_maxbins_10_value_extent[0] ===bin_maxbins_10_value_extent[1], [bin_maxbins_10_value_extent[0], bin_maxbins_10_value_extent[0]*1.1], bin_maxbins_10_value_extent)

[kanitw]

Instead of writing crazy signals in Vega-Lite, I think we better fix this in Vega. Will send a PR.

[domoritz]

Closing in favor of vega/vega#1700.