Activity Heatmap (Vega-Lite)

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "data": {
    "values": [
      {"day": "Mon", "hour": "9", "v": 3}, {"day": "Mon", "hour": "12", "v": 7}, {"day": "Mon", "hour": "18", "v": 5},
      {"day": "Tue", "hour": "9", "v": 4}, {"day": "Tue", "hour": "12", "v": 8}, {"day": "Tue", "hour": "18", "v": 6},
      {"day": "Wed", "hour": "9", "v": 6}, {"day": "Wed", "hour": "12", "v": 9}, {"day": "Wed", "hour": "18", "v": 4}
    ]
  },
  "mark": "rect",
  "encoding": {
    "x": {"field": "hour", "type": "ordinal", "title": "Hour"},
    "y": {"field": "day", "type": "ordinal", "title": null},
    "color": {"field": "v", "type": "quantitative", "scale": {"scheme": "greens"}, "title": "Activity"}
  }
}