A line chart displays average car horsepower per year, with an overlaid scatterplot of individual data points, each circle colored by car model. The average trend is shown as a blue line, while hovering over any point reveals a tooltip with the model, year, and horsepower. The visualization aggregates the CSV data using `d3.rollup` and computes the mean horsepower, then renders the SVG with `d3.line` for the average line and `d3.scaleOrdinal` for color mapping. Interactive tooltips are enabled through `d3.select` and mouse event handlers.