# What are Sankey Diagrams?

1. A Sankey diagram is a directional flow chart where the width of the streams is proportional to the quantity of flow, and where the flows can be combined, split and traced through a series of events or stages.

There are several key terms:

• A node is a stage in the flow.
Nodes are potential points of transformation, where flows may combine, diverge or detour.
The number of nodes is user defined.
At a minimum, a Sankey diagram requires 2 nodes: a source (or input) node where flows begin, and a sink (or output) node where flows end.
• Each node has two edges
An input edge that is defined by each of the flows that are entering the node, and an output edge that is defined by each of the flows that are exiting the node.
• Each edge of a node must have a weight.
The weight is a quantity, expressed in whatever units have been selected for quantifying flows for the whole diagram.
The weight is used to establish the width of each flow entering or exiting a node.
The total input edge flows for a node should equal the total output edge flows to create a balanced flow.
2.

3. Sankey Diagrams are directed, weighted graphs.

The focus is on weight functions that satisfy flow conservation:

The sum of the incoming weights equals the sum of the outgoing weights at all nodes except sources and sinks.
Such a graph is particularly appropriate for representing flow through a system in that it gives an immediate visualization of the weight of the flows and thus enables priority areas to be identified and tackled.
Examples include:

• Traffic flow in a city
• Energy flow in a production plant
• Water flow through a network of vessels and pipes
• Internet packet flow
• Call diversion in a call center
• The flow of money through a national economy
• and so on….

Check out the examples created with SDraw