Bar chart that highlights values beyond a threshold

To create a bar chart that highlights values beyond a threshold, we use two layers of bar marks. The lower layer shows all the bars while the upper layer shows bar with values above the threshold in red (#e45755). We then layer a rule mark and a text mark over the bars to annotate the threshold value

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Vega-Lite JSON Specification

{
  "$schema": "https://vega.github.io/schema/vega-lite/v2.json",
  "description": "The PM2.5 value of Beijing observed 15 days, highlighting the days when PM2.5 level is hazardous to human health. Data source https://chartaccent.github.io/chartaccent.html",
    "layer": [{
      "data": {
        "values": [
          {"Day": 1, "Value": 54.8},
          {"Day": 2, "Value": 112.1},
          {"Day": 3, "Value": 63.6},
          {"Day": 4, "Value": 37.6},
          {"Day": 5, "Value": 79.7},
          {"Day": 6, "Value": 137.9},
          {"Day": 7, "Value": 120.1},
          {"Day": 8, "Value": 103.3},
          {"Day": 9, "Value": 394.8},
          {"Day": 10, "Value": 199.5},
          {"Day": 11, "Value": 72.3},
          {"Day": 12, "Value": 51.1},
          {"Day": 13, "Value": 112.0},
          {"Day": 14, "Value": 174.5},
          {"Day": 15, "Value": 130.5}
        ]
      },
      "layer": [{
        "mark": "bar",
        "encoding": {
          "x": {"field": "Day", "type": "ordinal", "axis": {"labelAngle": 0}},
          "y": {"field": "Value", "type": "quantitative"}
        }
      }, {
        "mark": "bar",
        "transform": [
          {"filter": "datum.Value >= 300"},
          {"calculate": "300", "as": "baseline"}
        ],
        "encoding": {
          "x": {"field": "Day", "type": "ordinal"},
          "y": {"field": "baseline", "type": "quantitative"},
          "y2": {"field": "Value", "type": "quantitative"},
          "color": {"value": "#e45755"}
        }
      }
    ]}, {
      "data": {
         "values": [
          {"ThresholdValue": 300, "Threshold": "hazardous"}
        ]
      },
      "layer": [{
        "mark": "rule",
        "encoding": {
          "y": {"field": "ThresholdValue", "type": "quantitative"}
        }
      }, {
        "mark": {
          "type": "text",
          "align": "right",
          "dx": -2,
          "dy": -4
        },
        "encoding": {
          "x": {
            "value": "width"
          },
          "y": {
            "field": "ThresholdValue",
            "type": "quantitative",
            "axis": {"title": "PM2.5 Value"}
          },
          "text": {"field": "Threshold", "type": "ordinal"}
        }
      }]
    }
  ]
}