Conveying Flow: The Art and Science of Sankey Chart Design

In the world of data visualization, Sankey diagrams have long been a cornerstone for illustrating the flow of energy, mass, or value in processes or systems. First conceptualized by Edward Joseph sagar Sankey in the late 19th century, Sankey diagrams are a specialized and efficient way to depict the magnitude of flows between multiple processes, especially for the study of energy transfer systems. As information design evolves, the art and science of Sankey chart creation have gained in importance and complexity. This article explores the principles behind Sankey chart design, the tools available for creating them, and their diverse applications across various disciplines.

Understanding the Basics of SankeyCharts

Sankey charts are a type of flow diagram in which the width of arrows represents the relative magnitude of the flow between processes, resources, or entities. Each process in a Sankey diagram is depicted as a “step,” and the overall structure resembles a river that splits into channels, with the flow of the main river shown by the width of the channels.

The key principles of Sankey chart design include:

1. Flow Direction: The flow should be indicated consistently (usually from left to right) across the diagram, with individual flows easily distinguishable.

2. Width Representation: The width of each arrow is directly proportional to the quantity of flow passing through it. This makes it easy to compare the rates of flow at a glance.

3. Efficiency: The aim of a Sankey chart is to represent the efficiency of the entire system, with a narrowing of the channels at points where energy or material is lost or transformed into a less useful form.

Tools for Creating Sankey Charts

Tools for constructing Sankey diagrams have evolved from handmade sketches to more sophisticated software applications. Here are some of the main tools available:

1. Inkscape and Python Libraries: For users with a flair for coding, Inkscape, an open-source vector graphics editor, can be used in tandem with Python libraries such as matplotlib.sankey and pythonista. This combination allows for precise control over the chart’s configuration.

2. Sankey.js: An open-source library for JavaScript that allows users to create interactive and high-quality Sankey diagrams in web browsers.

3. Excel and Data Visualization Software: Advanced versions of Excel and other data visualization software such as Tableau and Power BI include Sankey diagramming capabilities, making it accessible to non-programmers.

Applications of Sankey Charts

Sankey diagrams have a wide range of applications across various disciplines, including:

1. Energy Efficiency Analysis: In the field of energy systems, Sankey diagrams are invaluable for illustrating the efficiency of energy flow. They can be used to evaluate the performance of power plants, heating systems, and renewable energy systems.

2. Environmental Workflow Monitoring: They are also used in environmental science to track materials and waste flows, such as in waste recycling or industrial wastewater treatment processes.

3. Financial Services: In sectors like banking and insurance, Sankey charts can be applied to illustrate the flow of funds or the movement of products and services.

4. Biological Systems: Sankey diagrams can be used in biology and medicine to visualize the flow of substances, such as metabolites or blood flow.

5. Business and Economics: They can illustrate the economic flow of value within a company, highlighting where value is being added and where it is being lost.

The Art of Designing Sankey Charts

Creating an effective Sankey chart requires more than just technical proficiency. It involves a delicate balance between art and science, as users must consider:

1. Clarity and Simplicity: The chart should be easy to read and understand, avoiding unnecessary complexity that could confuse the viewer.

2. Appropriateness of Scale: The scale used for the arrows must be appropriate for representing the magnitude of flows accurately.

3. Color Use: Colors should be chosen carefully to enhance readability and to convey the story you are trying to tell—whether through highlighting significant processes or conveying the direction of flow.

4. Contextual Information: Providing context through a legend, explanatory notes, or annotations is essential for users to fully understand the diagram.

5. Interactive Elements: In some cases, turning a Sankey chart into an interactive element can provide a richer viewer experience and allow deeper analysis.

Sankey charts are a powerful tool in the data visualization arsenal. Whether you are analyzing the energy consumption in a building, tracing the flow of financial transactions, or monitoring the material flows through a production line, understanding the principles and applications of Sankey charts will empower you to make more informed decisions and better communicate the flow of systems to others.

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