This heatmap visualizes carrier delay counts across months and years for U.S. airlines, using color intensity to show delay severity (from light to dark blue). Built with D3 v7, it maps data to a filled grid of SVG rectangles, with year and month on the axes and hover tooltips for exact values. It includes a linear color legend and uses scales from `d3.scaleBand` and `d3.scaleLinear`, while the data comes from a CSV hosted on GitHub.