This example shows a D3.js area chart that visualizes the trend of average car prices over the years, with a gradient-filled area below the line and interactive tooltips on hover. Data is loaded from a CSV file using `d3.csv`, transformed with `d3.rollups` to compute yearly means, and sorted by year before rendering. The visualization uses SVG elements with `d3.area` and `d3.line` generators, linear scales for the x- and y-axes, and a linear gradient definition for the area fill. Circles for each data point trigger tooltips showing the year and average price.