This example demonstrates a refactoring pattern known as the “viz function,” which encapsulates configurable logic for graphics and visualization. It uses d3.select to manage an SVG container, computing inner dimensions from margins and rendering a gray rectangle via d3’s data join pattern. The main module calls the viz function with parameters for margins, dimensions, and fill color, showing how reusable visualization code can be structured. The code relies on D3 version 7.8.5 and is licensed under MIT.