Sankey chart

(quoted from Sankey chart definition in Wikipedia)

Sankey charts are a type of flow diagram in which the width of the arrows is proportional to the flow rate.

The illustration shows a sankey chart that represents all the primary energy that flows into a factory. The widths of the bands are linearly proportional to energy production, utilization and loss. The primary energy inputs enter the left side of the diagram, and are differentiated into gas, electricity and coal/oil.

Sankey diagrams can also visualize the energy accounts, material flow accounts on a regional or national level, and cost breakdowns, and emphasize the major transfers or flows within a system. They help locate the most important contributions to a flow. They often show conserved quantities within defined system boundaries.

Charba provides out of the box the feature to enable sankey chart, leveraging on Chart.js Sankey.

The Chart.js Sankey is native javascript implementation and Charba provides the wrapper in order to be able to use it.

Programmatically, you could use a bar chart as following:

// creates the chart    
SankeyChart chart = new SankeyChart();
// adds to DOM
component.add(chart);
...
// example for Elemental2
// gets the chart instance as DOM element
Element element = chart.getChartElement().as();
// adds to DOM
DomGlobal.document.body.appendChild(element);

By UIBinder (ONLY for GWT), you could use a bar chart as following:

<ui:UiBinder xmlns:ui="urn:ui:com.google.gwt.uibinder"
   xmlns:g="urn:import:com.google.gwt.user.client.ui"
   xmlns:c="urn:import:org.pepstock.charba.client.gwt.widgets">

   <g:HTMLPanel  width="100%">
      ....
      <c:SankeyChartWidget ui:field="chart"/>
      ...
   </g:HTMLPanel>
</ui:UiBinder> 

Dataset

The sankey chart allows to define the data and a number of properties, used to display the data, by a sankey dataset.

Every chart has got a method to create a typed dataset accordingly with the chart type. The dataset can be also created instantiating the constructor.

// creates the chart
SankeyChart chart = new SankeyChart();
// creates the dataset
SankeyDataset dataset = chart.newDataset();
// sets the option
dataset.setColorMode(ColorMode.GRADIENT);
...
// creates the dataset
SankeyDataset datasetNew = new SankeyDataset();
// sets the option
datasetNew.setColorMode(ColorMode.GRADIENT);
...
// sets the dataset to the chart
chart.getData().setDatasets(dataset);

The following are the attributes that you can set:

Name	Type	Default	Scriptable	Description
borderColor	String - IsColor	HtmlColor.BLACK -	-	The color of the node border.
borderWidth	int	1	-	The stroke width of the sankey nodes in pixels.
color	String - IsColor	HtmlColor.BLACK -	-	The color of the node label.
colorFrom	String[] - IsColor[]	empty list	Yes	The starting color of the flow between nodes.
colorMode	ColorMode	ColorMode.GRADIENT	Yes	How the flow is drawn on the chart.
colorTo	String[] - IsColor[]	empty list	Yes	The ending color of the flow between nodes.
font	IsFont	Defaults.get().getGlobal() .getFont()	Yes	Font of text of node label. See Font.
labels	Labels	null	-	Is a map to apply a different label to the nodes.
nodeWidth	int	10	-	The width of node.
padding	double	font.getLineheight() / 2	Yes	The padding of the node label from the top.
priority	Priority	null	-	Is a map to apply a different priority to the nodes, affecting the chart layout.
size	Size	Size.MAX	Yes	Sets Size.MIN if flow overlap is preferred.

Scriptable

Scriptable options at dataset level accept a callback which is called for each of the underlying data values. See more details in Configuring charts section.

// creates chart
SankeyChart chart = new SankeyChart();
// creates dataset
SankeyDataset dataset = chart.newDataset();
// sets the option by a callback 
dataset.setColorFrom(new ColorCallback<DatasetContext>(){

   @Override
   public IsColor invoke(DatasetContext context){
      // logic
      return color;
   }
});

The following options can be set by a callback:

Name	Callback	Returned types
colorFrom	ColorCallback<DatasetContext>	String - IsColor
colorMode	ColorModeCallback	ColorMode
colorTo	ColorCallback<DatasetContext>	String - IsColor
font	FontCallback<DatasetContext>	FontItem
padding	PaddingCallback	double
size	SizeCallback	Size

Data structure

The data of a dataset for a sankey chart can be passed in sankey data points.

caution

The setData method is available but you can NOT use them otherwise an exception will throw.
Use setDataPoints instead.

Data as objects

Sankey data should be provided by a list of objects. The sankey data point contains the relations between nodes and their value.

It needs to have from and to values, which represent the nodes, and value which represents the value of the sankey element.

// creates chart
SankeyChart chart = new SankeyChart();
// creates dataset
SankeyDataset dataset = chart.newDataset();
// creates a list of data points
List<SankeyDataPoint> datapoints = dataset1.getDataPoints(true);
datapoints.add(new SankeyDataPoint("a", "b", 20));
datapoints.add(new SankeyDataPoint("c", "d", 10));
datapoints.add(new SankeyDataPoint("c", "e", 5));
// stores datapoints to dataset
dataset.setDataPoints(datapoints);

Options

The sankey chart defines specific options implementation to be configured.

// creates chart
SankeyChart chart = new SankeyChart();
// gets the chart options
SankeyOptions options = chart.getOptions();
// sets option
options.setResponsive(true);

info

The sankey chart disables the DATALABELS, LABELS, ZOOM, ANNOTATION, HTML legend, color schemes and dataset items selector plugins and the options can NOT be set globally but only at chart level.

Scales

The sankey chart can use cartesian linear axes and they are not showed.