This example shows a simple bar chart of letter frequencies from a local CSV file. The bars are interactive, so clicking one toggles its selection, highlighting it in a darker color while dimming the others. Built with React and D3.js v7, it uses a declarative `join` pattern to render the SVG elements, `scaleBand` and `scaleLinear` for the axes, and a `ResizeObserver` in React to make the chart responsive to container size changes.