A donut chart in React and D3 renders four slices with a teal-to-black color gradient, generated by `d3.interpolateRgb` and drawn using `d3.pie` and `d3.arc`. React components (`Slice`) map over the pie data to produce SVG `<path>` elements within a centered `<g>` transform. A separate, commented-out `Check` component demonstrates a linear sequence of gradient rectangles using the same interpolation approach.