/documd-visuals

Error Bar — Team Variance Around Mean (Vega-Lite)

Best for: showing a central estimate and its spread when the reader cares about uncertainty but not the full boxplot shape Avoid when: you need the full distribution or every raw point matters on the page Answers: what the mean is, and how wide the variability around it appears for each category

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  "$schema": "https://vega.github.io/schema/vega-lite/v6.json",
  "width": 430,
  "height": 260,
  "title": {"text": "Team Variance Around Mean", "subtitle": "Average review time with standard-deviation bars", "anchor": "start"},
  "data": {
    "values": [
      {"team": "Core", "hours": 18}, {"team": "Core", "hours": 21}, {"team": "Core", "hours": 24},
      {"team": "Billing", "hours": 23}, {"team": "Billing", "hours": 30}, {"team": "Billing", "hours": 36},
      {"team": "Identity", "hours": 14}, {"team": "Identity", "hours": 16}, {"team": "Identity", "hours": 19}
    ]
  },
  "layer": [
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      "mark": {"type": "errorbar", "ticks": true},
      "encoding": {
        "x": {"field": "team", "type": "nominal", "title": null},
        "y": {"field": "hours", "type": "quantitative", "title": "Hours"}
      }
    },
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      "encoding": {
        "x": {"field": "team", "type": "nominal"},
        "y": {"aggregate": "mean", "field": "hours", "type": "quantitative"}
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Data Shape

One row per observation. Vega-Lite derives the error bar from the grouped raw values while a second layer marks the mean explicitly.

Key Options

Pitfalls

• ❌ Reading error bars as full distribution shape → ✅ they summarize spread, not quartiles and outliers
• ❌ Omitting the center estimate → ✅ the reader still needs the anchor value, not just the interval |
• ❌ Comparing bars built from incompatible assumptions → ✅ be clear whether the spread is stdev, stderr, or CI |

Alternatives