Tooltip issue

This is a postscript to https://bl.ocks.org/ijlyttle/84ce12ce2c1093070f645a2bae3b8fa0

The x scale is UTC and works as I expect; a solution is to add a UTC scale to the tooltip:

    "tooltip": [
      {
        "field": "date",
        "type": "temporal",
        "format": "%Y-%m-%d",
        "scale": {
          "type": "utc"
        }
      },
      {
        "field": "value",
        "type": "quantitative"
      }
    ]