This heatmap visualizes earthquake counts across U.S. states from 2010 to 2015, with each cell colored by magnitude category. The view initially filters to the "10 to 99" category, but a dropdown menu lets viewers switch between magnitude ranges. Built with D3 v5, the code uses `d3.scaleBand` for axes, `d3.scaleQuantize` for color mapping, and `d3.nest` for category grouping. SVG rectangles render the data, with tooltips showing exact counts via mouseover and mouseleave events.