The Sankey diagram

This year I stumbled across a diagram I had never seen before: a Sankey diagram. The example I saw was in a short documentary, effectively plotting energy flows from source to end-user (DW Planet A, 2024), as a data visualisation to show how energy is generated... and how much is wasted in the process. As the documentary writers pointed out, an ICE engine wastes 80% of the petrol energy in heat, exhaust and noise, whereas an EV only wastes 20% of its energy (DW Planet A, 2024, 6:13). We can map both processes via Sankey diagrams: from raw materials to waste, or "rejected energy" (0:56).

So a Sankey diagram is a graphic representation, or statistical graphics - like pie and bar charts - presenting quantitative data (or data sets) in an understandable form so we can clearly grasp what the authors are trying to tell us (Vos & Frejd, 2022). They "are flow charts, in which the width of flows is proportional to the quantity" (Vos & Frejd, 2022, p. 290), used to plot movements "of people over time and space, [such as] human migration across continents, and military movements in warfare" (p. 291).

Sankey diagrams are a mathematical structure, and are "models of real-world phenomena"; and should arise from a "truthful and accurate" representation of the underlying data (Vos & Frejd, 2022, p. 293). Check out the Vos and Frejd paper: they have some great examples of Sankey diagrams, including Norwegian population flows (see image below, p. 292), and dairy product mapping from cow to consumer.

They are named after an Irish Captain, one Matthew Henry Phineas Riall Sankey (he must have had rich parents, to have been given FOUR names!). Captain Sankey created the first example of what is now known as the Sankey diagram in 1898, mapping the energy of a steam engine (Kennedy & Sankey, 1898, Plate 5; see graphic accompanying this post). 

We can create our own Sankey diagrams, at no cost, from the GitHub powered web platform at https://sankeymatic.com/ (Bogart, 2024). 

The whole thing is quite clever.

Sam

References:

DW Planet A. (2024, June 1). The mind-blowing thing we get WRONG about energy [video]. YouTube. https://youtu.be/EVJkq4iu7bk

Bogart, S. (2024). Home. Sankeymatic. https://sankeymatic.com/

Kennedy, A. B. W., & Sankey, M. H. P. R. (1898). The Thermal Efficiency Of Steam Engines. Minutes of the Proceedings of the Institution of Civil Engineers, 134(1), 278-312. https://doi.org/10.1680/imotp.1898.19100

Vos, P., & Frejd, P. (2022). Grade 8 Students Appropriating Sankey Diagrams: The First Cycle in an Educational Design Research. Journal on Mathematics Education, 13(2), 289-306. https://files.eric.ed.gov/fulltext/EJ1379624.pdf