While there may be some debate about the exact definitions and differences between Sankey diagrams and Alluvial diagrams, both types of diagrams are powerful tools for visualizing complex data and relationships.
At their core, both Sankey and Alluvial diagrams are designed to show the flow of data or information between different categories or dimensions. Sankey diagrams are often used to represent the movement of resources, energy, or money between different stages or processes, while Alluvial diagrams are more commonly used to show how a population or group of individuals can be categorized and sub-categorized based on different criteria.
One key difference between the two types of diagrams is the way in which they are read and interpreted. As mentioned earlier, Sankey diagrams are typically read from left to right, with the flow of information moving in a single direction. Alluvial diagrams, on the other hand, are more flexible in terms of how they can be read and interpreted. In an Alluvial diagram, the nodes and categories can be arranged in any order or orientation, and the connections between them can be read in any direction.
Another important factor to consider when choosing between Sankey and Alluvial diagrams is the specific type of data or information you are trying to represent. Sankey diagrams are often used to represent continuous or quantitative data, such as the flow of water through a system or the movement of money between bank accounts. Alluvial diagrams, on the other hand, are more commonly used to represent categorical or qualitative data, such as the distribution of demographic groups within a population or the breakdown of customer preferences by product category.
The reality is that there are no consensus about the difference between Sankey diagrams and Alluvial diagrams definitions.
Some, such as RAWGraphs, argue that alluvial diagrams are a sub-category belonging into the family of the Sankey diagrams. According to them "they use the same logic to show how the same set of items regroups according to different dimensions." Wikipedia articles about Sankey diagrams and Alluvial diagrams seem to go in the same direction.
Others, such as DataSmith, argue that Alluvial diagrams are mostly used to represents how a population splits, looking at different characteristics of the individuals, while Sankey diagram displays a flow. This implies that Sankey diagrams must be read from left to right to be interpreted correctly, while this is not true for Alluvial diagrams.
Lastly, Azavea and a few others argue that the two terms can be used interchangeabilly.
And us ? What do we think about it at Sankey Journey ? We would argue that it does not really matter, as most of the people usually hear the word 'Sankey ' or 'Alluvial' for the first time when they are shown a beautiful diagram, so there is not much risk of confusion. Now, looking at what users are actually using the most, there is a clear winner. Sankey is the most popular word used according to Google trends, and this is why our free online tool to generate users flow is called Sankey Journey!
Ultimately, the choice between Sankey and Alluvial diagrams will depend on the specific needs of your project and the type of data you are working with. Both types of diagrams can be incredibly useful for visualizing complex data and relationships, and there is no one "right" or "wrong" way to use them. As with any data visualization tool, the key is to choose the type of diagram that best fits your needs and helps you tell the story you want to tell with your data.
We hope this article helped you seeing clearer with all these definitions! And by the way, if you wan to create your own Sankey/Alluvial diagram to visualize how your users are using your app or your website, give it a try!