A dropdown menu lets users select a country, and a line chart displays that country’s population over time. The visualization uses D3.js to parse CSV data, render SVG axes and paths, and animate transitions between selections. A categorical color scale from `d3-scale-chromatic` assigns a distinct stroke color to each country’s line, while `d3.scaleLinear` maps years and population values to coordinates. The interface updates the line via a `transition` when the selection changes.