Flowing Ideas: The Art and Science of Sankey Charts

Sankey diagrams, also known as Sankey flow diagrams or Sankey matrices, are a powerful visualization tool used to represent quantities flow from one process or system to another. This distinctive chart type is named after Mark Barr, a former employee of International Business Machines (IBM), who is believed to have developed it around 1940. Despite its relatively recent historical origins, Sankey diagrams have become widely used in various fields including economics, engineering, and environmental science, among others, due to their ability to effectively communicate complex data. In this article, we delve into the creation of Sankey charts, explore their applications, and discuss the art and science behind these visually engaging representations.

Understanding the Structure of Sankey Charts

At its core, a Sankey chart is a directed flow diagram consisting of several flow steps, typically represented by arrows flowing from left to right. Each arrow’s width or thickness corresponds to the magnitude of the data it represents. This makes Sankey diagrams particularly effective for illustrating processes with a high volume of data that require a large number of steps. The chart is anchored at two points: the source and the sink—the starting and end points of the flow, respectively. The direction of the flow is usually from left to right, which means that the source of the flow is on the left, and the final destination or sink is on the right.

Creating a Sankey Chart: An Overview

Creating a Sankey chart involves several steps, both in data preparation and in the graphical representation of that data. Here’s a simplified overview of the process:

Data Collection and Preparation: Start by collecting the raw data you wish to visualize. This data must be in a tidy format, which means each row represents an observation, and each column represents a variable.
Summarize Data: Since Sankey diagrams require the data to be summarized to a certain level, ensure your data is aggregated in a way that supports a clear, understandable flow.
Choose Your Sankey Creation Tool: There are numerous tools available, ranging from spreadsheet applications (e.g., Excel) to specialized visualization software (e.g., Tableau, Python libraries like Matplotlib and Plotly, R packages like ggplot2 and dplyr).
Organize Your Data: Structure your data in a tabular format, typically with three columns: the source, the target, and the quantity of the flow between them.
Create Your Sankey Chart: Use the chosen tool to input your data and format your chart according to the principles of good visualization design. Consider factors like color coding, labels, font sizes, and the direction of the flows.
Review and Refine: After generating your chart, walk through it with the audience you intend to share it with. Refine based on feedback to ensure the visual story it tells is clear and accurate.

Applications of Sankey Charts

Sankey diagrams are particularly useful for illustrating a wide range of processes, including environmental flows (energy, water, emissions), financial flows, product life cycles, and supply chains. Their ability to display the magnitude and distribution of quantities makes them invaluable for:

Understanding Resource Flows: Whether it’s analyzing energy use in buildings, water consumption in a city, or the allocation of project resources, Sankey diagrams help track and understand these flows.
Visualizing Economic Flows: In finance and economics, Sankey diagrams are used to illustrate trade flows between countries, the distribution of wealth, or the economic impact of certain policies.
Improving Supply Chain Visibility: Businesses leverage Sankey diagrams to visualize the flow of materials through their supply chains, aiding in process optimization and efficiency.
Educational Purposes: In classrooms and educational materials, Sankey diagrams are used to explain complex concepts in a way that is both engaging and understandable to learners.

The Art and Science of Sankey Charts

The creation and application of Sankey diagrams hinge on both art and science. The art of Sankey charting lies in the ability to make quantitative data visually appealing and informative, engaging the viewer’s interest and encouraging them to explore the data further. The science, however, is in the accurate representation of the data, ensuring that the flow chart accurately reflects the real-world processes or phenomena being studied. When executed effectively, Sankey charts serve as tools that enhance understanding, aid decision-making, and facilitate the transfer of knowledge.

Conclusion

Sankey diagrams are a versatile and visually compelling tool that offer a unique way to visualize flows and transitions. While their construction requires careful consideration of data preparation, format, and presentation, the rewards of clarity, engagement, and insight make the effort well worth it. As technology and visualization tools continue to evolve, the role of Sankey charts in communicating complex information is likely to grow, making them an indispensable part of the data visualization toolkit for years to come.

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