Flowing Facts: The Magic Behind Sankey’s Colorful Data Art

Data visualization is a cornerstone of modern analysis and communication, with various tools and techniques serving the purpose of unveiling patterns, trends, and connections in information that would otherwise be lost or misunderstood in raw data. One such tool that has stood the test of time is the Sankey diagram, a type of flow diagram introduced in the 19th century and still in wide use today. Renowned for its ability to transform complex data streams into stunning, fluid illustrations, the Sankey diagram is both an artistic and an analytical treasure. This article explores the world of Sankey charts—how they are created, and the numerous applications that make them indispensable for experts and enthusiasts alike.

The Originator: William Playfair

William Playfair, a 19th-century economist and engineer, is credited with the invention of the Sankey diagram back in 1857. He developed this versatile graphic method to illustrate the energy efficiency of various systems, and over the years, it has been adapted and refined to illustrate various flows, such as material, cost, water, and human resources.

The Visual Grammar of Sankey Diagrams

Sankey diagrams have a unique structure—a river-like flow that branches out at various points according to the quantity of material or energy being transported—a feature that makes them instantly recognizable. There are several components to understand when creating or interpreting a Sankey chart:

Streamlines: Represent the flow of data. The width of the streamline indicates the quantity of the substance or energy being transported.
Nodes: Where streamline branches intersect; they symbolize points of interest, like events, processes, activities, or sources and sinks in a network of connectivity.
Directions: Sankey diagrams flow from the source to the destination, emphasizing where energy or material is lost or transformed.
Heat Map Visualization: Sometimes Sankey diagrams are embedded within larger heat maps to show the distribution or concentration of data at various points.

How to Create a Sankey Diagram

The process of creating a Sankey diagram is similar to building a puzzle. Here’s a simple guide:

Define the System: Understand the system or process the Sankey diagram is designed to represent. Determine the inputs and outputs of the system.
Outline the Nodes: Identify the key points within your system—all relevant inputs, processes, and outputs.
Determine Streamline Widths: Based on the data, assign a width to each streamline based on the magnitude of the material or energy flowing through it.
Design the Streamlines: Connect the nodes using streamline paths, ensuring that the widths represent the relative flow. It helps to maintain consistent angles for more readable diagrams.
Label and Annotate: Clearly label the streamlines and nodes, include a legend for any colors used, and add a description if needed.
Iterate for Clarity: The first attempt may not be perfect. Iterate and tune the design for clarity and understanding.

Applications of Sankey Diagrams

Sankey diagrams’ unparalleled ability to demonstrate the flow and distribution of data across vast systems has led to their adoption in several fields, including:

Energy flows: Illustrating the efficiency and losses in energy systems such as power plants.
Financial flows: Tracking the flow of money in businesses or government budgets.
Material flows: Representing the movement of raw materials through manufacturing processes.
Transportation and logistics: Demonstrating routes, traffic flows, or supply chain efficiency.
Ecological systems: Mapping the flow of nutrients, energy, or organisms through ecosystems.
Water systems: Visualizing the flow of water through different stages of treatment and reuse.

Conclusion

Sankey diagrams are more than just colorful data art—they are windows into the often invisible flows and exchanges that happen within complex systems. By utilizing their unique properties, we can analyze, explain, and even predict the behavior of systems from manufacturing lines to global finance. As data science continues to grow in importance, the art and utility of the Sankey diagram are likely to become even more celebrated, bridging the gap between complex ideas and visual simplicity.

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