Visual Map of Data

Data visualization is a procedure of mapping data into visual elements. This procedure can also be called visual coding, and visual elements can also be called visual channels.

Every type of charts in Apache ECharts^TM has this built-in mapping procedure. For example, line chart map data into lines, bar chart map data into height. Some more complicated charts, like graph, themeRiver, and treemap have their own built-in mapping.

Besides, ECharts provides visualMap component for general visual mapping. Visual elements allowed in visualMap component are:

• symbol, symbolSize
• color, opacity, colorAlpha,
• colorLightness, colorSaturation, colorHue

Next, we are going to introduce how to use visualMap component.

Data and Dimension

Data are usually stored in series.data in ECharts. Depending on chart types, like list, tree, graph, and so on, the form of data may vary somehow. But they have one common feature, that they are a collection of data items. Every data item contains data value, and other information if needed. Every data value can be a single value (one dimension) or an array (multiple dimensions).

For example, series.data is the most common form, which is a list, a common array:

series: {
  data: [
    {
      // every item here is a dataItem
      value: 2323, // this is data value
      itemStyle: {}
    },
    1212, // it can also be a value of dataItem, which is a more common case
    2323, // every data value here is one dimension
    4343,
    3434
  ];
}

series: {
  data: [
    {
      // every item here is a dataItem
      value: [3434, 129, 'San Marino'], // this is data value
      itemStyle: {}
    },
    [1212, 5454, 'Vatican'], // it can also be a value of dataItem, which is a more common case
    [2323, 3223, 'Nauru'], // every data value here is three dimension
    [4343, 23, 'Tuvalu'] // If is scatter chart, usually map the first dimension to x axis,
    // the second dimension to y axis,
    // and the third dimension to symbolSize
  ];
}

Usually the first one or two dimensions are used for mapping. For example, map the first dimension to x axis, and the second dimension to y axis. If you want to represent more dimensions, visualMap is what you need. Most likely, scatter charts use radius to represent the third dimension.

The visualMap Component

visualMap component defines the mapping from which dimension of data to what visual elements.

The following two types of visualMap components are supported, identified with visualMap.type.

Its structure is defined as:

option = {
  visualMap: [
    // can define multiple visualMap components at the same time
    {
      // the first visualMap component
      type: 'continuous' // defined as continuous visualMap
      // ...
    },
    {
      // the second visualMap component
      type: 'piecewise' // defined as discrete visualMap
      // ...
    }
  ]
  // ...
};

Continuous and Piecewise Visual Mapping Components

The visual mapping component of ECharts is divided into continuous (visualMapContinuous) and piecewise (visualMapPiecewise).

Continuous means that the data dimension for visual mapping is a continuous value, while piecewise means that the data is divided into multiple segments or discrete data.

Continuous Visual Mapping

Continuous type visual mapping can determine the range of visual mapping by specifying the maximum and minimum values.

option = {
  visualMap: [
    {
      type: 'continuous',
      min: 0,
      max: 5000,
      dimension: 3, // the fourth dimension of series.data (i.e. value[3]) is mapped
      seriesIndex: 4, // The fourth series is mapped.
      inRange: {
        // The visual configuration in the selected range
        color: ['blue', '#121122', 'red'], // A list of colors that defines the graph color mapping
        // the minimum value of the data is mapped to 'blue', and
        // the maximum value is mapped to 'red', // the maximum value is mapped to 'red', // the maximum value is mapped to 'red'.
        // The rest is automatically calculated linearly.
        symbolSize: [30, 100] // Defines the mapping range for the graphic size.
        // The minimum value of the data is mapped to 30, // and the maximum value is mapped to 100.
        // The maximum value is mapped to 100.
        // The rest is calculated linearly automatically.
      },
      outOfRange: {
        // Check the out of range visual configuration
        symbolSize: [30, 100]
      }
    }
    // ...
  ]
};

where visualMap.inRange indicates the style used for data within the data mapping range; while visualMap.outOfRange specifies the style for data outside the mapping range.

visualMap.dimension specifies which dimension of the data will be visually mapped.

Piecewise Visual Mapping

The piecewise visual mapping component has three modes.

• Continuous data average segmentation: based on visualMap-piecewise.splitNumber to automatically split the data into pieces equally.
• Continuous data custom segmentation: define the range of each piece based on visualMap-piecewise.pieces.
• Discrete data (categorical data): categories are defined in visualMap-piecewise.categories.

To use segmented visual map, you need to set type to 'piecewise' and choose one of the above three configuration items.