select

Overview

The select mark transform is a very flexible and efficient selection. It groups data by specified channels and selects data from each group according to specified channels and selectors. Through the select transform, you can filter data based on conditions and implement marking at specific positions.

Use Cases

Peak Annotation: Mark the highest/lowest points in line charts
Start/End Annotation: Highlight start/end points in time series
Category Annotation: Differentiate marks for different data categories
Trend Annotation: Mark statistical feature points like mean, median, etc.

Configuration

Property	Description	Type	Default
groupBy	Group by specified channels	`string` \| `string[]`	`series`
channel	For each group, use specified channel for data extraction	Channel
selector	For each group, specify the corresponding data extraction operation	Selector	`first`

channel

Theoretically, channel can be set to all channel values. For details, please refer to the encode documentation. All enumeration values are as follows:

type Channel =
  | 'x'
  | 'y'
  | 'z'
  | 'x1'
  | 'y1'
  | 'series'
  | 'color'
  | 'opacity'
  | 'shape'
  | 'size'
  | 'key'
  | 'groupKey'
  | 'position'
  | 'series'
  | 'enterType'
  | 'enterEasing'
  | 'enterDuration'
  | 'enterDelay'
  | 'updateType'
  | 'updateEasing'
  | 'updateDuration'
  | 'updateDelay'
  | 'exitType'
  | 'exitEasing'
  | 'exitDuration'
  | 'exitDelay'
  | `position${number}`;

selector

type Selector =
  | 'min' // Minimum value
  | 'max' // Maximum value
  | 'first' // First value
  | 'last' // Last value
  | 'mean' // Mean value
  | 'median' // Median value
  | ((I: number[], V: number[]) => number[]); // Custom selection function

Example

As shown below, add value annotations to the top of bar charts:

import { Chart } from '@antv/g2';

const data = [
  { letter: 'A', frequency: 0.08167 },
  { letter: 'B', frequency: 0.01492 },
  { letter: 'C', frequency: 0.02782 },
  { letter: 'D', frequency: 0.04253 },
  { letter: 'E', frequency: 0.12702 },
  { letter: 'F', frequency: 0.02288 },
  { letter: 'G', frequency: 0.02015 },
  { letter: 'H', frequency: 0.06094 },
  { letter: 'I', frequency: 0.06966 },
  { letter: 'J', frequency: 0.00153 },
  { letter: 'K', frequency: 0.00772 },
  { letter: 'L', frequency: 0.04025 },
  { letter: 'M', frequency: 0.02406 },
  { letter: 'N', frequency: 0.06749 },
  { letter: 'O', frequency: 0.07507 },
  { letter: 'P', frequency: 0.01929 },
  { letter: 'Q', frequency: 0.00095 },
  { letter: 'R', frequency: 0.05987 },
  { letter: 'S', frequency: 0.06327 },
  { letter: 'T', frequency: 0.09056 },
  { letter: 'U', frequency: 0.02758 },
  { letter: 'V', frequency: 0.00978 },
  { letter: 'W', frequency: 0.0236 },
  { letter: 'X', frequency: 0.0015 },
  { letter: 'Y', frequency: 0.01974 },
  { letter: 'Z', frequency: 0.00074 },
];

const chart = new Chart({
  container: 'container',
});
chart.options({
  width: 800,
  paddingLeft: 50,
  paddingRight: 100,
  data,
  children: [
    // Bar chart mark configuration
    {
      type: 'interval',
      encode: {
        x: 'letter',
        y: 'frequency',
      },
    },
    // Text mark configuration
    {
      type: 'text',
      encode: {
        x: 'letter',
        y: 'frequency',
        text: 'frequency',
        series: 'frequency',
      },
      transform: [
        {
          // Use select transform mark
          type: 'select',
          // Based on y channel
          channel: 'y',
          // Select the highest point
          selector: 'max',
        },
      ],
      style: {
        // Annotation text offset 12 pixels to the left
        dx: -12,
        // Annotation text offset 12 pixels upward
        dy: -12,
      },
    },
  ],
});

chart.render();