This heatmap visualizes weekly U.S. retail gasoline and diesel prices from 1995 to 2021, with columns representing different fuel grades and formulations, and rows representing weekly dates. Color intensity encodes price levels, using a viridis color scale. Built with D3.js v7, the visualization renders as an SVG using `d3.scaleBand` for axes, `d3.timeFormat` for date parsing, and `d3.interpolateViridis` for color mapping. A tooltip, triggered by mouseover events, displays the column description, date, and value, while a gradient legend shows the color-to-price correspondence.