Sankey diagram

Definition

Flow diagram using edges and arrows with proportional width

Summary

A Sankey diagram visualizes the flow of quantities (originally energy, but it can also be mass, money or any other quantity) through a system by using arrows or paths whose width is proportional to the corresponding quantity. Named after Irish captain Matthew Henry Phineas Riall Sankey, who used this type of diagram in 1898 to present energy flows in a steam engine.

Anatomy

• Nodes: Rectangular blocks or points representing entities, stages, or categories in the system
• Links (or flows): Connecting paths between nodes whose width represents the magnitude of flow
• Direction: Flows typically move from left to right or top to bottom, indicated by the path direction
• Width encoding: The thickness of each link is proportional to the quantity being transferred
• Color: Often used to trace specific flows through the system or to distinguish different categories
• Labels: Text annotations identifying nodes and sometimes showing numerical values of flows

Interpreting a sankey diagram

To read a Sankey diagram, start by identifying the source nodes (typically on the left or top) and follow the flows to their destinations (right or bottom). The width of each flow path directly corresponds to its magnitude: wider paths represent larger quantities. The principle of (energy) conservation is verified when the total width entering a node equals the combined width of paths leaving it.

When and how to use a sankey diagram

Strengths

• Reveals the relative magnitude of flows at a glance through visual encoding
• Shows conservation and transformation of quantities through a system
• Identifies dominant pathways and significant losses or gains
• Displays complex many-to-many relationships in a comprehensible format
• Highlights inefficiencies or bottlenecks in processes

Caveats and limitations

• Becomes cluttered and difficult to read with too many nodes or crossing flows
• Can be misleading if flows are not properly scaled or if the conservation principle is violated
• Difficult to compare multiple Sankey diagrams side by side
• May require significant space to display clearly, especially for complex systems
• Not suitable for showing temporal changes or trends over time (unless using animated versions)

Use cases

• Energy flow analysis showing conversion, losses and consumption
• Budget allocation and financial flows through organizations
• Resource consumption and waste generation in manufacturing

See also

• Why Sankey diagrams are so great for understanding energy data

• Sankey diagrams now have the new Python package they deserved

Links

Wikidata entity: Q1667530 (Sankey diagram)

Wikipedia page: Sankey diagram