Visualize data with class breaks

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This sample demonstrates how to visualize features based on numeric data using manually defined class breaks. When breaks are known or predefined, this renderer provides options for setting a distinguishing symbol for each class break.

Prior to completing the following steps, you should be familiar with views, Map, and FeatureLayer. If necessary, complete the following tutorials first:

The basic components of this app, such as creating instances of the Map and MapView classes and understanding HTML and CSS structure will not be reviewed. See the tutorials listed above if you need to familiarize yourself with those components in this application. As a general rule the introductory principles discussed in the tutorials above apply to most samples in the documentation.

1. Create a symbol for each class break

This application uses data representing U.S. Census block groups in Seattle, WA. The demographic of interest is the number of adults ages 25+ who have a college degree. Our users may have predefined breaks they want to use for visualizing this data, such as areas were 0 - 35% of the population has a degree, 35% - 50%, 50% - 75%, and 75% - 100%.

First, we'll define a separate symbol for each break using SimpleFillSymbol.

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const less35 = {
  type: "simple-fill", // autocasts as new SimpleFillSymbol()
  color: "#fffcd4",
  style: "solid",
  outline: {
    width: 0.2,
    color: [255, 255, 255, 0.5]
  }
};

const less50 = {
  type: "simple-fill", // autocasts as new SimpleFillSymbol()
  color: "#b1cdc2",
  style: "solid",
  outline: {
    width: 0.2,
    color: [255, 255, 255, 0.5]
  }
};

const more50 = {
  type: "simple-fill", // autocasts as new SimpleFillSymbol()
  color: "#38627a",
  style: "solid",
  outline: {
    width: 0.2,
    color: [255, 255, 255, 0.5]
  }
};

const more75 = {
  type: "simple-fill", // autocasts as new SimpleFillSymbol()
  color: "#0d2644",
  style: "solid",
  outline: {
    width: 0.2,
    color: [255, 255, 255, 0.5]
  }
};

2. Create an instance of ClassBreaksRenderer

We can use a ClassBreaksRenderer when defining class breaks for features. In the constructor we specify the field and normalizationField, which contains the data of interest.

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const renderer = {
  type: "class-breaks", // autocasts as new ClassBreaksRenderer()
  field: "COL_DEG", // total number of adults (25+) with a college degree
  normalizationField: "EDUCBASECY", // total number of adults 25+
  defaultSymbol: {
    type: "simple-fill", // autocasts as new SimpleFillSymbol()
    color: "black",
    style: "backward-diagonal",
    outline: {
      width: 0.5,
      color: [50, 50, 50, 0.6]
    }
  },
  defaultLabel: "no data" // legend label for features that don't match a class break
};

3. Set symbol on each class break

You can set a symbol on each class break in one of two ways: Using classBreakInfos in the constructor...

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const renderer = {
  type: "class-breaks", // autocasts as new ClassBreaksRenderer()
  // other properties set in step number 2
  classBreakInfos: [
    {
      minValue: 0,
      maxValue: 0.3499,
      symbol: less35,
      label: "< 35%" // label for symbol in legend
    },
    {
      minValue: 0.35,
      maxValue: 0.4999,
      symbol: less50,
      label: "35 - 50%" // label for symbol in legend
    },
    {
      minValue: 0.5,
      maxValue: 0.7499,
      symbol: more50,
      label: "50 - 75%" // label for symbol in legend
    },
    {
      minValue: 0.75,
      maxValue: 1.0,
      symbol: more75,
      label: "> 75%" // label for symbol in legend
    }
  ]
};

Or with the addClassBreakInfo() method.

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renderer.addClassBreakInfo(0, 0.3499, less35);

4. Summary

Once the renderer is defined, you can set it on the layer and the view and legend will automatically update. Click the sandbox button below to see the full code of the app.

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