This example demonstrates how to define the boundaries of a visualization using margins, rendering a fixed gray inner rectangle on an SVG canvas. The outer container dimensions are read directly from the page, then margin values create a centered plotting area of a specified size. The code uses D3.js’s `select` and `join` methods to append the SVG and its rectangle, calculating the `innerWidth` and `innerHeight` by subtracting the margins from the container size. This setup ensures space is reserved for future axes or chart elements.