This example shows an interactive bar chart of a small CSV dataset mapping letters to counts. The React app manages a state object for the selected letter, while D3 v7 renders the SVG, including axes, bars, and a title. Clicking a bar toggles its selection, changing its fill color and opacity. The chart uses D3’s `scaleBand` for the x-axis, `scaleLinear` for the y-axis, and React’s `useEffect` and `useRef` to coordinate state and dimensions.