Flowing with Data: The Art and Science of Sankey Charts

Sankey diagrams are a unique way of representing data flow, often described as a type of flow diagram. They are effective tools for visualizing complex systems where multiple inputs produce a variety of outputs. Originating in the mid-19th century, these charts have evolved from their use in engineering and scientific data analysis to widespread application in a broad spectrum of fields, from economics to environmental studies. This article delves into the art and science behind creating and interpreting Sankey diagrams, and highlights their diverse applications.

The Science of Sankey Charts

At their core, Sankey diagrams are graphical representations of how flow, or quantities, transform between multiple states. This transformation is usually represented by the width of streams, with the width varying directly with the quantity of flow. The creation of a Sankey diagram involves several steps:

Identifying the Inputs: Clearly define the initial sources or inputs. These are often the starting points of the transformation process, such as raw materials in manufacturing or sources of energy in a power plant.
Identifying the Transformations or Outputs: After the initial inputs, the data can undergo multiple transformations or end up at multiple outputs. Each of these outputs should be listed and categorized.
Quantifying the Flow: Once the inputs and outputs are identified, a numerical value is assigned to each, indicating the amount (expressed in kg, L, kWh) or quantity (number of units) that represents the flow from one state to another.
Designing the Chart: Sankey diagrams are typically constructed with arrow-shaped streams or beams. The width of each stream visually represents the magnitude of the flow from one state to another. The width of the beam increases or decreases proportionally with the quantity of flow.
Finalizing and Refining: The final step involves refining and adjusting the widths of the streams based on the relative magnitudes of the flow. This step is crucial for ensuring that the chart is visually balanced and accurately represents the data.

The Art of Interpreting Sankey Diagrams

While the scientific method and data are the foundation of Sankey diagrams, their interpretation is somewhat of an art. Here are some tips for reading and understanding Sankey diagrams:

Analyze the Widths: The width of each stream indicates the relative amount of flow. A wider stream signifies a larger quantity or value moving from one state to another.
Look for Changes: Notice any significant shifts in the flow between states. This can often indicate changes in efficiency, losses, or gains in the system.
Look for Trends: Over time, trends in the width and direction of streams can reveal patterns or changes in the data.

Applications of Sankey Charts

Sankey diagrams are used across a wide range of fields, from engineering and finance to environmental studies and data analysis. Some of the most common uses include:

Sustainability and Climate Change: Sankey diagrams are powerful tools for illustrating the carbon footprint of different products or transportation methods. They can also show energy flows through buildings or the lifecycle of a product, highlighting efficiency improvements or areas for environmental savings.
Supply Chain Analysis: These diagrams can visualize the flow of materials through different stages, identifying bottlenecks, inefficiencies, or cost savings opportunities.
Financial Analysis: In finance, Sankey diagrams can trace the movement of money through different accounts, projects, or investment strategies, helping financial managers make strategic decisions.
Educational and Research Outreach: Sankey diagrams are excellent educational tools, allowing complex systems to be visualized and simplified, making them accessible to a broader audience.

Conclusion

Sankey diagrams are much more than just a visual tool; they are a powerful representation of complex data. By visually depicting the flow and transformation of data, they offer a unique lens through which to understand and communicate intricate processes, systems, and their impacts. Whether it’s in the hands of a data scientist mapping out energy efficiency improvements or an educational outreach program illustrating the carbon footprint of different energy sources, Sankey diagrams are an invaluable tool for demystifying complexity and making data more accessible and understandable.

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