This example maps a four-by-four grid of Cartesian coordinates to an SVG rectangle, using React components and D3.js v5. It renders a single red rectangle at a position derived from the looping variables `i` and `j`. The code defines reusable React components like `MyRect` and `MyCircle`, and uses `ReactDOM.render` to display the SVG. The approach demonstrates basic x/y positioning within a fixed 960×500 viewport.