This example shows a gray rectangle positioned inside a larger SVG canvas, demonstrating how margins define the "inner rectangle" where data marks are placed while reserving space for axes. The code uses D3's `select` and data joins to create an SVG element sized to the container, then draws a single rectangle with fixed margins of 50 pixels on all sides. The inner dimensions are calculated by subtracting these margins from the full width and height, and the rectangle is filled with a light gray color to visually highlight the plotting area. This straightforward setup illustrates the foundational layout pattern used before adding data-driven visual elements.