This example visualizes yearly infection counts for US states from 1970 to 1980 using a heatmap. Cells are colored on a red scale, with darker shades representing higher infection numbers, and hover interactions reveal a tooltip with state, year, and exact count. The visualization uses d3.csv to load data, appends SVG text elements for labels and legend, and creates colored rect elements for each data point. The tooltip is positioned with event.pageX and event.pageY, while the legend is drawn manually with a loop generating gradient rectangles.