lineY

Overview

The lineY and lineX marks have similar configurations. The lineY mark is used to draw auxiliary lines perpendicular to the y-axis, commonly used for drawing average values or other aggregated data auxiliary lines.

Data Configuration Methods

lineY supports two methods for configuring data:

Method 1: Direct data Configuration (Recommended for Simple Scenarios)

When you need to draw horizontal lines at fixed positions, you can directly configure data as a number array. G2 will automatically convert the array to y channel encoding.

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

const chart = new Chart({
  container: 'container',
});

chart.options({
  type: 'view',
  height: 200,
  autoFit: true,
  children: [
    {
      type: 'interval',
      data: [
        { year: '1951', sales: 38 },
        { year: '1952', sales: 52 },
        { year: '1956', sales: 61 },
        { year: '1957', sales: 120 },
        { year: '1958', sales: 48 },
        { year: '1959', sales: 38 },
      ],
      encode: { x: 'year', y: 'sales' },
    },
    {
      type: 'lineY',
      data: [100, 59], // Automatically converted to encode: { y: [100, 59] }
      style: {
        stroke: (v) => {
          if (v >= 60) {
            return 'green';
          } else {
            return 'red';
          }
        },
        lineWidth: 2,
      },
      labels: [
        // Text configuration can refer to mark text
        {
          text: (v) => (v >= 60 ? 'lineY divider 1' : 'lineY divider 2'),
          position: 'top-right',
          textBaseline: 'bottom',
          fill: '#000',
          fillOpacity: 0.45,
          background: true,
          backgroundFill: '#000',
          backgroundOpacity: 0.15,
        },
      ],
    },
  ],
});

chart.render();

Method 2: Explicit encode Configuration (For Complex Data Processing)

When you need to work with data fields or combine with data transforms, you should explicitly configure the encode channels. This method is more flexible and supports data field mapping and various data transforms.

/**
 * A recreation of this demo: https://vega.github.io/vega-lite/examples/layer_precipitation_mean.html
 */
import { Chart } from '@antv/g2';

const chart = new Chart({
  container: 'container',
  autoFit: true,
});

chart.options({
  type: 'view',
  data: {
    type: 'fetch',
    value: 'https://assets.antv.antgroup.com/g2/seattle-weather.json',
  },
  children: [
    {
      type: 'interval',
      transform: [{ type: 'groupX', y: 'mean' }],
      encode: { x: (d) => new Date(d.date).getUTCMonth(), y: 'precipitation' },
      scale: { y: { tickCount: 5, domainMax: 6 } },
      tooltip: { channel: 'y', valueFormatter: '.2f' },
    },
    {
      type: 'lineY',
      transform: [{ type: 'groupX', y: 'mean' }],
      encode: { y: 'precipitation' },
      style: {
        stroke: '#F4664A',
        strokeOpacity: 1,
        lineWidth: 2,
        lineDash: [3, 3],
      },
    },
  ],
});

chart.render();

Configuration Method Comparison

Configuration Method	Use Case	Example	Auto Conversion
`data: [value1, value2]`	Fixed position auxiliary lines	`data: [50, 100]`	✅ Auto convert to `encode.y`
`encode: { y: field }`	Based on data fields or transforms	`encode('y', 'fieldName')`	❌ Explicit configuration

Important Notes:

When both data array and encode.y are configured, encode.y takes higher priority
data auto conversion only works when array elements are simple values (not objects)
When using transform for data aggregation, you must use the encode method

For more examples, you can visit the Chart Examples - Line Annotation page.

Configuration

Property	Description	Type	Default	Required
data	Configure the data for the `lineY` mark, supports number arrays (automatically converted to y channel) or object arrays	Array	-
encode	Configure the visual channels of the `lineY` mark, including `y`, `color`, etc., to specify the relationship between visual element properties and data	encode	-	✓
style	Configure the graphic style of the `lineY` mark	style	-

encode

Configure the visual channels of the lineY mark.

Property	Description	Type	Default	Required
y	Bind the `y` property channel of the `lineY` mark. Supports data field mapping or works with transforms to calculate aggregated values (such as mean or median). Note: When directly configuring `data` as an array, it will be automatically converted to the `y` channel, no need to configure again.	encode	-	✓
color	Bind the `color` property channel of the `lineY` mark. If a data field is mapped to the color channel, the data will be grouped and split into multiple regions with different colors	encode	-

style

Property	Description	Type	Default
stroke	Stroke of the graphic	string \| Function<string>	-
strokeOpacity	Stroke opacity	number \| Function<number>	-
lineWidth	Width of the graphic stroke	number \| Function<number>	-
lineDash	Dashed stroke configuration. The first value is the length of each dash segment, the second value is the distance between segments. Setting lineDash to [0, 0] results in no stroke	[number,number] \| Function<[number, number]>	-
opacity	Overall opacity of the graphic	number \| Function<number>	-
shadowColor	Shadow color of the graphic	string \| Function<string>	-
shadowBlur	Gaussian blur coefficient of the graphic shadow	number \| Function<number>	-
shadowOffsetX	Horizontal distance of the shadow from the graphic	number \| Function<number>	-
shadowOffsetY	Vertical distance of the shadow from the graphic	number \| Function<number>	-
cursor	Mouse cursor style. Same as CSS cursor style, default ‘default’	string \| Function<string>	`default`

Examples

Multiple Threshold Lines

Shows how to draw multiple threshold lines with different styles in the same chart, commonly used for data monitoring and alert systems.

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

const chart = new Chart({
  container: 'container',
  autoFit: true,
});

// Simulate time series data
const data = Array.from({ length: 30 }, (_, i) => ({
  day: i + 1,
  value: 50 + Math.sin(i * 0.3) * 20 + Math.random() * 10,
}));

chart.options({
  type: 'view',
  data,
  children: [
    // Draw the main line chart
    {
      type: 'line',
      encode: { x: 'day', y: 'value' },
      style: { stroke: '#1890ff', lineWidth: 2 },
    },
    // Draw data points
    {
      type: 'point',
      encode: { x: 'day', y: 'value' },
      style: { fill: '#1890ff', r: 3 },
    },
    // Danger threshold line (red)
    {
      type: 'lineY',
      data: [75],
      style: {
        stroke: '#ff4d4f',
        strokeOpacity: 0.8,
        lineWidth: 2,
        lineDash: [8, 4],
      },
      labels: [
        {
          text: 'Danger Threshold',
          position: 'top-right',
          fill: '#ff4d4f',
          fontWeight: 'bold',
          background: true,
          backgroundFill: '#fff',
          backgroundOpacity: 0.9,
        },
      ],
    },
    // Warning threshold line (orange)
    {
      type: 'lineY',
      data: [65],
      style: {
        stroke: '#fa8c16',
        strokeOpacity: 0.8,
        lineWidth: 2,
        lineDash: [5, 5],
      },
      labels: [
        {
          text: 'Warning Threshold',
          position: 'top-right',
          fill: '#fa8c16',
          fontWeight: 'bold',
          background: true,
          backgroundFill: '#fff',
          backgroundOpacity: 0.9,
        },
      ],
    },
    // Normal threshold line (green)
    {
      type: 'lineY',
      data: [35],
      style: {
        stroke: '#52c41a',
        strokeOpacity: 0.8,
        lineWidth: 2,
        lineDash: [3, 3],
      },
      labels: [
        {
          text: 'Normal Lower Limit',
          position: 'bottom-right',
          fill: '#52c41a',
          fontWeight: 'bold',
          background: true,
          backgroundFill: '#fff',
          backgroundOpacity: 0.9,
        },
      ],
    },
  ],
});

chart.render();