Quantile-quantile chart

Use a quantile-quantile chart (Q-Q plot) to check how closely a sample follows a normal distribution: points near the line are normal-like, points that bend away are not.

Most deliveries track a normal curve, but the slowest few fall well above it

Days to deliver against the normal quantile; the dashed line passes through the quartiles

This chart cannot be displayed here: it needs JavaScript and WebGL.

x is the standard-normal quantile at plotting position (rank - 0.5) / n; the dashed line runs through the first and third quartile pairs

The same chart in each style

Source

Open in playground

Rich Markdown

```chart
type: qq
title: Most deliveries track a normal curve, but the slowest few fall well above it
subtitle: Days to deliver against the normal quantile; the dashed line passes through the quartiles
data:
  inline:
    - {days: 2.1}
    - {days: 2.6}
    - {days: 2.9}
    - {days: 3.1}
    - {days: 3.3}
    - {days: 3.4}
    - {days: 3.6}
    - {days: 3.7}
    - {days: 3.9}
    - {days: 4.0}
    - {days: 4.2}
    - {days: 4.3}
    - {days: 4.5}
    - {days: 4.6}
    - {days: 4.9}
    - {days: 5.1}
    - {days: 5.4}
    - {days: 5.8}
    - {days: 6.3}
    - {days: 7.2}
    - {days: 8.9}
encoding: {y: days}
axes:
  y: {label: Days to deliver}
```

Fields that matter

type: qq with encoding.y for the numeric sample, one value per row
encoding.color draws one set of points and one line per group; encoding.facet splits the chart into panels
x is the standard-normal quantile at plotting position (rank - 0.5) / n; the dashed reference line runs through the first and third quartile pairs
Readouts give the sample value as you wrote it, its theoretical quantile and its rank; RichView computes the last two, so they are marked as derived
Rows without a value are listed as excluded; text that is not a number is an error
axes.y sets label, format and unit; the chart is square by default (style.ratio changes it)

Interactions

Hover or focus a point to read its value, quantile and rank.
Click a legend entry to toggle a group when the chart has several.
Open the Table view, Download PNG or CSV, or Expand the chart.