Global Energy Flow , Source to End Use, Width = Exajoules Figure 62.1 , Width = Exajoules

A Sankey diagram encodes directed flows between nodes using ribbons whose width is strictly proportional to the flow quantity. Nodes are rectangles (or points) arranged in columns representing stages of a process; ribbons connect source nodes to target nodes and can split or merge at each stage. The perceptual mechanism is area: the viewer's eye integrates ribbon width over length, making proportional flow comparisons automatic , a ribbon that is twice as wide carries twice the quantity, and this is legible without reading any number.

The dataset is a directed multi-stage flow system , energy moving from primary sources through conversion stages to end uses with explicit losses , and the message is about where quantity is concentrated and where it is dissipated. No other chart type encodes this simultaneously. A stacked bar chart at each stage could show totals but cannot show which source flows to which end use. A chord diagram would show bilateral relationships but imposes a circular layout that loses the left-to-right process directionality.

An alluvial diagram is the closest structural relative. The distinction is subtle but important: alluvial diagrams show how categorical composition changes across time or grouping variables , the flows represent reclassifications of the same population. Sankey diagrams show physical or financial flows where quantity is transferred, not just reclassified, and losses are permitted (total output at a node can be less than total input, encoding inefficiency or waste).

FRAMEWORK FT Visual Vocabulary category: Flow , "Show the volume or intensity of movement between two or more states or conditions." Tufte principle: the Sankey is data-ink efficient when the flow quantity is the message , every pixel of ribbon width encodes a real quantity. The one design decision worth knowing: node horizontal position is fixed by stage column, but vertical position within a column is determined by the layout algorithm minimising link crossings.

Paste this into Claude Code to generate a working version of this chart, plus its data file. The result will not be a perfect replica , the goal is that the reader can run the prompt, get a chart of this type, and read its source. Generate a complete, self-contained sankey diagram in D3 v7. Two files:

The ideas in this chapter didn't appear from nowhere. Matthew Henry Phineas Riall Sankey drew the diagram that bears his name in 1898 , for an engineering analysis of energy losses in a steam engine, where flow widths encoded thermal-energy magnitudes. Energy-balance diagrams still use his form a century later. Matthew Henry Phineas Riall Sankey, circa 1898. AI-generated portrait based on a public domain photograph (Wikimedia Commons).

The ideas in this chapter didn't appear from nowhere. Matthew Henry Phineas Riall Sankey drew the diagram that bears his name in 1898 , for an engineering analysis of energy losses in a steam engine, where flow widths encoded thermal-energy magnitudes. Energy-balance diagrams still use his form a century later. Matthew Henry Phineas Riall Sankey, circa 1898. AI-generated portrait based on a public domain photograph (Wikimedia Commons).