Sankey charts are a specialized type of data visualization that provide a unique perspective on data flow. These charts are particularly useful for understanding and communicating complex systems where items or entities move between different states or categories. They are named after the Scottish engineer Matthew Henry Phineas Riall Sankey, who used them in the mid-1800s to illustrate the energy transformations in a steam engine’s process.
## What Are Sankey Charts?
Sankey charts consist of one or more parallel bands that represent flow lines. Each band’s width corresponds to the volume or quantity of the represented flow, making it visually intuitive to understand the distribution and magnitude of flows. These charts are often used in contexts such as energy consumption, material balance in industrial processes, financial transactions, or information flows in networks.
## Components of a Sankey Chart
### Source and Sink
In a Sankey diagram, “source” nodes represent the origins of the flow, while “sink” nodes represent the destinations where the flows end. These nodes are typically located at one end of the chart.
### Flow Lines and Bands
Flow lines connect the source and sink nodes and are usually labeled to indicate the nature or type of the flow (e.g., transactions by sector, energy consumption by resource). Bands within these lines correspond to the different flow volumes or categories, each represented by varying colors and widths.
### Labels and Annotations
Labels for flow lines help clarify what each flow represents. Additional annotations or data labels can provide specific values or percentages along the lines to enhance readability and understanding.
### Stacking
In cases where there are many categories or sources/sinks, the bands within the flow lines are stacked, with each stack representing a different category in the flow.
## Creating a Sankey Chart
### Data Preparation
To create a Sankey chart, you need to prepare a structured data set. The essential components include:
– **From (Source Node)**: The origin of each flow. This could be a category, location, or identifier. It is a mandatory field.
– **To (Sink Node)**: The destination of each flow. This also can be a category, location, or identifier, and it must match the source node to prevent loops.
– **Weight (Flow Volume)**: The quantity or value that you want to represent. This field is often represented by the width of the bands in the chart.
### Tools for Creation
#### Software & Applications
There are several tools used to create Sankey charts:
– **Microsoft Excel**: Offers basic Sankey chart creation through custom shape and VBA code.
– **Tableau**: Provides built-in Sankey chart capabilities for data visualization, making it versatile for use with any data format.
– **R and Python**: With packages like `sankey`, `plotly`, or `networkx`, you can create highly customizable Sankey diagrams programmatically.
– **D3.js**: A JavaScript library for creating SVG and Canvas-based diagrams, suitable for web-based complex visualizations.
#### Step-by-Step Instructions
1. **Data Preparation**: Clean and organize your data set, ensuring that all categories or sources and sinks are correctly identified.
2. **Choosing a Tool**: Decide on the tool that aligns best with the complexity of your data set and your proficiency level.
3. **Input Data**: Format your data set according to the requirements of your chosen tool, ensuring columns correspond to source nodes, target nodes, and weights.
4. **Diagram Creation**: Use the tool’s features to create the Sankey chart. Adjust settings and formatting to optimize visual clarity and comprehensibility.
5. **Review and Adjust**: Assess the chart’s readability and effectiveness. Make necessary adjustments to labels, colors, and other elements to enhance understanding.
## Interpreting Sankey Charts
When interpreting Sankey charts, focus on:
– **Volume and Direction**: The width of the bands indicates the magnitude of flow, while their direction shows the movement (from source to sink).
– **Dynamics Exposed**: Look for patterns, such as whether flows are increasing, decreasing, or remain constant over time.
– **Category Comparisons**: Comparing the widths of bands within categories helps identify the most significant flows relative to others.
– **Trends and Shifts**: Tracking changes in the widths and directions of flows between different points can reveal evolving systems or inefficiencies.
## Conclusion
Sankey charts offer a graphical representation of how flows occur between different categories or nodes, making them an invaluable tool in the field of data visualization. By understanding their components, creation processes, and interpretation techniques, you can effectively use them in various applications from business insights to scientific research, improving communication of complex systems and relationships.
Whether you are an analyst, a scientist, or a data enthusiast, mastering the dynamics of data flows through Sankey charts enhances your ability to make informed decisions based on data, and to convey complex information in an engaging and understandable form.
