Sankey

Sankey diagrams are a type of flow diagram used to visualize the distribution of inputs and outputs within a system. The distinctive feature is that the width of each arrow is proportional to the magnitude of the flow it represents, allowing quick assessment of where resources are consumed or lost. The diagrams are named after Captain Matthew Henry Phineas Riall Sankey, who popularized the format in an 1898 illustration of a steam engine’s energy balance. Since then they have become a common tool in engineering, energy analysis, and environmental studies.

Construction and interpretation: A Sankey diagram consists of nodes or blocks connected by flows. On many diagrams, inputs are placed on the left and outputs on the right, with intermediate stages showing transformations or losses. Flows can represent energy quantities, mass, or monetary value, and are often color-coded by category. The total width of inputs equals the total width of outputs, reflecting conservation of the quantified resource. While horizontal layouts are common, diagrams can be vertical or multi-tiered to show multiple process steps.

Applications and limitations: Sankey diagrams are used to illustrate energy balances in power plants and industrial processes, material flows in manufacturing, water management, and life-cycle assessments. They are facilitated by software tools and libraries that support automated layout from data. However, diagrams can become cluttered with many streams, and the visual emphasis on width can obscure the relative importance of streams if not properly labeled. Clear legends, units, and scaling are essential for accurate interpretation.