This example demonstrates D3 data joins by binding a dataset of country-weight pairs to SVG elements across two canvases. It starts by selecting and restyling existing rectangles with D3’s `select` and `attr`, then appends circles manually before using `data()` and `join()` to automatically create new circles mapped from the array’s `size` and `weight` values. The code also shows data transformation with `forEach`, grouping countries into weight arrays, and renders everything as SVG in a React setup using D3 v7.