A Sankey diagram visualizes the flow of data through a REST API for a pet store, showing the relationships between API endpoints, HTTP methods, and response codes. The chart uses D3.js with the `d3.sankey` plugin to render the flow, with nodes representing resources like `PetResource` and links showing the connections between paths. The visualization is interactive, allowing users to drag nodes to rearrange the layout. Data is loaded from static JSON files containing node and link definitions with associated values.