A Sankey diagram is a visualization used to depict a flow from one set of values to another. The things being connected are called nodes and the connections are called links. Sankeys are best used when you want to show a many-to-many mapping between two domains (e.g., universities and majors) or multiple paths through a set of stages (for instance, Google Analytics uses Sankeys to show how traffic flows from pages to other pages on your web site).
For the curious, they're named after Captain Sankey, who created a diagram of steam engine efficiency that used arrows having widths proportional to heat loss.
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