Calculate Percentage of Total

{
  "$schema": "https://vega.github.io/schema/vega-lite/v2.json",
  "description": "A bar graph showing what activites consume what percentage of the day.",
  "data": {
    "values": [
      {"Activity": "Sleeping","Time": 8},
      {"Activity": "Eating","Time": 2},
      {"Activity": "TV","Time": 4},
      {"Activity": "Work","Time": 8},
      {"Activity": "Exercise","Time": 2}
    ]
  },
  "transform": [{
    "window": [{
        "op": "sum",
        "field": "Time",
        "as": "TotalTime"
    }],
    "frame": [null, null]
  },
  {
    "calculate": "datum.Time/datum.TotalTime * 100",
    "as": "PercentOfTotal"
  }],
  "mark": "bar",
  "encoding": {
    "x": {
      "field": "PercentOfTotal",
      "type": "quantitative",
      "axis": {
        "title": "% of total Time"
      }
    },
    "y": {
      "field": "Activity",
      "type": "nominal",
      "scale": {
        "rangeStep": 12
      }
    }
  }
}