This line chart visualizes the average calories burned by age, using data from a lifestyle CSV file. The code aggregates individual records by age and calculates the mean calories burned, then plots the resulting trend as a smooth, steel-blue line. Built with D3 v7, it employs `d3.csv` and `d3.autoType` for data loading, `d3.rollup` for aggregation, and `d3.scaleLinear` for axes. The SVG rendering uses `d3.axisBottom` and `d3.axisLeft` for ticks, and a `d3.line` path for the trend, with custom labels for both axes and the title.