Sankey Diagram

Overview

A sankey diagram is a visualization used to depict a flow from one set of values to another. The things being connected are called nodes and the connections are called links. Sankeys are best used when you want to show a many-to-many mapping between two domains (e.g., universities and majors) or multiple paths through a set of stages (for instance, Google Analytics uses sankeys to show how traffic flows from pages to other pages on your web site).

For the curious, they're named after Captain Sankey, who created a diagram of steam engine efficiency that used arrows having widths proportional to heat loss.

Note: The sankey chart may be undergoing substantial revisions in future Google Charts releases.

Sankey diagrams are rendered in the browser using SVG or VML, whichever is appropriate for the user's browser. Google's sankey layout code is derived from D3's sankey layout code.

Note: Google sankey charts are unavailable in Microsoft Internet Explorer 8 and earlier versions.

A Simple Example

Suppose you had two categories, A and B, that connect to three other categories, X, Y, and Z. Some of those connections are heavier than others. For instance, B has a thin connection to X and a much thicker connection to Y.


Try hovering over one of the links to highlight the connection.

To create a sankey chart, provide a set of rows, with each containing information about one connection: from, to, and weight. Then use the google.visualization.Sankey() method to initialize the chart and then the draw() method to render it:

<html>
  <head>
    <script type="text/javascript" src="https://www.gstatic.com/charts/loader.js"></script>
    <script type="text/javascript">
      google.charts.load('current', {'packages':['sankey']});
      google.charts.setOnLoadCallback(drawChart);

      function drawChart() {
        var data = new google.visualization.DataTable();
        data.addColumn('string', 'From');
        data.addColumn('string', 'To');
        data.addColumn('number', 'Weight');
        data.addRows([
          [ 'A', 'X', 5 ],
          [ 'A', 'Y', 7 ],
          [ 'A', 'Z', 6 ],
          [ 'B', 'X', 2 ],
          [ 'B', 'Y', 9 ],
          [ 'B', 'Z', 4 ]
        ]);

        // Sets chart options.
        var options = {
          width: 600,
        };

        // Instantiates and draws our chart, passing in some options.
        var chart = new google.visualization.Sankey(document.getElementById('sankey_basic'));
        chart.draw(data, options);
      }
    </script>
  </head>
  <body>
    <div id="sankey_basic" style="width: 900px; height: 300px;"></div>
  </body>
</html>

Note: Avoid cycles in your data: if A links to itself, or links to B which links to C which links to A, your chart will not render.

Multilevel Sankeys

You can create a Sankey chart with multiple levels of connections:

Sankey charts will add additional levels as needed, laying them out automatically. Here's the complete code for the above chart:

<html>
<body>
 <script type="text/javascript" src="https://www.gstatic.com/charts/loader.js"></script>

<div id="sankey_multiple" style="width: 900px; height: 300px;"></div>

<script type="text/javascript">
  google.charts.load("current", {packages:["sankey"]});
  google.charts.setOnLoadCallback(drawChart);
   function drawChart() {
    var data = new google.visualization.DataTable();
    data.addColumn('string', 'From');
    data.addColumn('string', 'To');
    data.addColumn('number', 'Weight');
    data.addRows([
       [ 'Brazil', 'Portugal', 5 ],
       [ 'Brazil', 'France', 1 ],
       [ 'Brazil', 'Spain', 1 ],
       [ 'Brazil', 'England', 1 ],
       [ 'Canada', 'Portugal', 1 ],
       [ 'Canada', 'France', 5 ],
       [ 'Canada', 'England', 1 ],
       [ 'Mexico', 'Portugal', 1 ],
       [ 'Mexico', 'France', 1 ],
       [ 'Mexico', 'Spain', 5 ],
       [ 'Mexico', 'England', 1 ],
       [ 'USA', 'Portugal', 1 ],
       [ 'USA', 'France', 1 ],
       [ 'USA', 'Spain', 1 ],
       [ 'USA', 'England', 5 ],
       [ 'Portugal', 'Angola', 2 ],
       [ 'Portugal', 'Senegal', 1 ],
       [ 'Portugal', 'Morocco', 1 ],
       [ 'Portugal', 'South Africa', 3 ],
       [ 'France', 'Angola', 1 ],
       [ 'France', 'Senegal', 3 ],
       [ 'France', 'Mali', 3 ],
       [ 'France', 'Morocco', 3 ],
       [ 'France', 'South Africa', 1 ],
       [ 'Spain', 'Senegal', 1 ],
       [ 'Spain', 'Morocco', 3 ],
       [ 'Spain', 'South Africa', 1 ],
       [ 'England', 'Angola', 1 ],
       [ 'England', 'Senegal', 1 ],
       [ 'England', 'Morocco', 2 ],
       [ 'England', 'South Africa', 7 ],
       [ 'South Africa', 'China', 5 ],
       [ 'South Africa', 'India', 1 ],
       [ 'South Africa', 'Japan', 3 ],
       [ 'Angola', 'China', 5 ],
       [ 'Angola', 'India', 1 ],
       [ 'Angola', 'Japan', 3 ],
       [ 'Senegal', 'China', 5 ],
       [ 'Senegal', 'India', 1 ],
       [ 'Senegal', 'Japan', 3 ],
       [ 'Mali', 'China', 5 ],
       [ 'Mali', 'India', 1 ],
       [ 'Mali', 'Japan', 3 ],
       [ 'Morocco', 'China', 5 ],
       [ 'Morocco', 'India', 1 ],
       [ 'Morocco', 'Japan', 3 ]
    ]);

    // Set chart options
    var options = {
      width: 600,
    };

    // Instantiate and draw our chart, passing in some options.
    var chart = new google.visualization.Sankey(document.getElementById('sankey_multiple'));
    chart.draw(data, options);
   }
</script>
</body>
</html>

Controlling Colors

Sankey charts have the ability to set custom colors for nodes and links. Both nodes and links can be given custom color palettes using their colors options (sankey.node.colors and sankey.link.colors, respectively). They can also be given different coloring modes using the colorMode option.

If the colors aren't customized, they default to the standard Material palette.

    var colors = ['#a6cee3', '#b2df8a', '#fb9a99', '#fdbf6f',
                  '#cab2d6', '#ffff99', '#1f78b4', '#33a02c'];

    var options = {
      height: 400,
      sankey: {
        node: {
          colors: colors
        },
        link: {
          colorMode: 'gradient',
          colors: colors
        }
      }
    };

You can control the colors of the links, nodes, and labels with configuration options. Here, we select three with the same hue but different brightnesses:

Here's what those options look like:

    var options = {
      width: 600,
      sankey: {
        link: { color: { fill: '#d799ae' } },
        node: { colors: [ '#a61d4c' ],
                label: { color: '#871b47' } },
      }
    };

You can also control the transparency of the links with the sankey.link.color.fillOpacity option:

    var options = {
      width: 600,
      sankey: {
        link: { color: { fill: '#d799ae', fillOpacity: 0.8 } },
        node: { colors: [ '#a61d4c' ],
                label: { color: '#871b47' } },
      }
    };

To create a border around the links, use the sankey.link.color.stroke and sankey.link.color.strokeWidth options:

The stroke color can be specified either in RGB format or by English name.

    var options = {
      width: 750,
      height: 400,
      sankey: {
        node: { colors: [ '#a61d4c' ] },
        link: { color: { stroke: 'black', strokeWidth: 1 } },
      }
    };

Customizing Labels

The text on sankey charts can be customized using sankey.node.label.fontName and friends:

Here's the option stanza for the above chart:

    var options = {
      width: 600,
      sankey: {
        node: { label: { fontName: 'Times-Roman',
                         fontSize: 14,
                         color: '#871b47',
                         bold: true,
                         italic: true } } },
    };

You can adjust the position of the labels relative to the nodes with the sankey.node.labelPadding option:

In the chart above, we've added 30 pixels of padding between the labels and the nodes.

    var options = {
      width: 600,
      sankey: { node: { labelPadding: 30 } },
    };

Adjusting Nodes

You can control the width of the nodes with sankey.node.width:

Above, we set the node width to 2.

    var options = {
      width: 600,
      sankey: { node: { width: 2 } },
    };

You can adjust the distance between the nodes with sankey.node.nodePadding:

In the above chart, we set sankey.node.nodePadding to 80.

    var options = {
      width: 900,
      sankey: { node: { nodePadding: 80 } },
    };

Loading

The google.charts.load package name is "sankey":

  google.charts.load("current" {packages: ["sankey"]});

The visualization's class name is google.visualization.Sankey:

  var visualization = new google.visualization.Sankey(container);

Data Format

Rows: Each row in the table represents a connection between two labels. The third column indicates the strength of that connection, and will be reflected in the width of the path between the labels.

Columns:

  Column 0 Column 1 Column 2 ... Column N (optional)
Purpose: Source Destination Value ... Optional roles
Data Type: string string number ...
Role: domain domain data ...
Optional column roles:

None

None

None

...

 

Configuration Options

Name
forceIFrame

Draws the chart inside an inline frame. (Note that on IE8, this option is ignored; all IE8 charts are drawn in i-frames.)

Type: boolean
Default: false
height

Height of the chart, in pixels.

Type: number
Default: height of the containing element
sankey.iterations

With multilevel sankeys, it's sometimes nonobvious where nodes should be placed for optimal readability. The D3 layout engine experiments with different node layouts, stopping when sankey.iterations attempts have been made. The larger this number, the more pleasing the layout of complex sankeys, but it comes with a cost: the sankeys will take longer to render. Conversely, the shorter this number, the quicker your charts will render.

Type: integer
Default: 32
sankey.link

Controls attributes of the connections between nodes. Currently all attributes pertain to color:

sankey: {
  link: {
    color: {
      fill: '#efd',     // Color of the link.
      fillOpacity: 0.8, // Transparency of the link.
      stroke: 'black',  // Color of the link border.
      strokeWidth: 1    // Thickness of the link border (default 0).
    },
    colors: [
      '#a6cee3',        // Custom color palette for sankey links.
      '#1f78b4',        // Nodes will cycle through this palette
      '#b2df8a',        // giving the links for that node the color.
      '#33a02c'
    ]
  }
}
      
Type: object
Default: null
sankey.link.colorMode

Sets a coloring mode for the links between nodes. Possible values:

  • 'source' - The color of the source node is used for the links to all target nodes.
  • 'target' - The color of the target node is used for the link to its source nodes.
  • 'gradient' - The link between a source and target node is colored as a gradient from the source node color to the target node color.
  • 'none' - the default option; link colors will be set to the default (or a color as specified by the sankey.link.color.fill and sankey.link.color.fillOpacity options).

This option overrides sankey.link.color.

Type: string
Default: 'none'
sankey.node

Controls attributes of the nodes (the vertical bars between links):

sankey: {
  node: {
    label: {
      fontName: 'Times-Roman',
      fontSize: 12,
      color: '#000',
      bold: true,
      italic: false
    },
    interactivity: true, // Allows you to select nodes.
    labelPadding: 6,     // Horizontal distance between the label and the node.
    nodePadding: 10,     // Vertical distance between nodes.
    width: 5,            // Thickness of the node.
    colors: [
      '#a6cee3',         // Custom color palette for sankey nodes.
      '#1f78b4',         // Nodes will cycle through this palette
      '#b2df8a',         // giving each node its own color.
      '#33a02c'
    ]
  }
}
      
Type: object
Default: null
sankey.node.colorMode

Sets a coloring mode for the sankey nodes. Possible values:

  • 'unique' - Each node will receive a unique color.
Type: string
Default: 'unique'
tooltip

An object with members to configure various tooltip elements. To specify properties of this object, you can use object literal notation, as shown here:

{textStyle: {color: '#FF0000'}, showColorCode: true}
Type: object
Default: null
tooltip.isHtml

If set to true, use HTML-rendered (rather than SVG-rendered) tooltips. See Customizing Tooltip Content for more details.

Note: customization of the HTML tooltip content via the tooltip column data role is not supported by the Bubble Chart visualization.

Type: boolean
Default: false
tooltip.textStyle

An object that specifies the tooltip text style. The object has this format:

{ color: <string>,
  fontName: <string>,
  fontSize: <number>,
  bold: <boolean>,
  italic: <boolean> }
    

The color can be any HTML color string, for example: 'red' or '#00cc00'. Also see fontName and fontSize.

Type: object
Default: {color: 'black', fontName: <global-font-name>, fontSize: <global-font-size>}
width

Width of the chart, in pixels.

Type: number
Default: width of the containing element

Methods

Method
draw(data, options)

Draws the chart. The chart accepts further method calls only after the readyevent is fired. Extended description.

Return Type: none
getBoundingBox(id)

Returns an object containing the left, top, width, and height of chart element id. The format for id isn't yet documented (they're the return values of event handlers), but here are some examples:

var cli = chart.getChartLayoutInterface();

Height of the chart area
cli.getBoundingBox('chartarea').height
Width of the third bar in the first series of a bar or column chart
cli.getBoundingBox('bar#0#2').width
Bounding box of the fifth wedge of a pie chart
cli.getBoundingBox('slice#4')
Bounding box of the chart data of a vertical (e.g., column) chart:
cli.getBoundingBox('vAxis#0#gridline')
Bounding box of the chart data of a horizontal (e.g., bar) chart:
cli.getBoundingBox('hAxis#0#gridline')

Values are relative to the container of the chart. Call this after the chart is drawn.

Return Type: object
getSelection()

Returns an array of the selected chart entities. Selectable entities are bars, legend entries and categories. For this chart, only one entity can be selected at any given moment. Extended description .

Return Type: Array of selection elements
setSelection()

Selects the specified chart entities. Cancels any previous selection. Selectable entities are bars, legend entries and categories. For this chart, only one entity can be selected at a time. Extended description .

Return Type: none
clearChart()

Clears the chart, and releases all of its allocated resources.

Return Type: none

Events

Name
error

Fired when an error occurs when attempting to render the chart.

Properties: id, message
onmouseover

Fired when the user mouses over a visual entity. Passes back the row and column indices of the corresponding data table element. A bar correlates to a cell in the data table, a legend entry to a column (row index is null), and a category to a row (column index is null).

Properties: row, column
onmouseout

Fired when the user mouses away from a visual entity. Passes back the row and column indices of the corresponding data table element. A bar correlates to a cell in the data table, a legend entry to a column (row index is null), and a category to a row (column index is null).

Properties: row, column
ready

The chart is ready for external method calls. If you want to interact with the chart, and call methods after you draw it, you should set up a listener for this event before you call the draw method, and call them only after the event was fired.

Properties: none
select

Fired when the user clicks a visual entity. To learn what has been selected, call getSelection().

Properties: none

Data Policy

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