This example shows a D3.js v4 heatmap that visualizes a matrix of values using the Inferno color scale, with an interactive tooltip that displays each cell’s exact value on hover. The chart loads CSV data from an external URL, builds band scales for the x and y axes, and renders colored rectangles with rounded corners. A title and subtitle are added via SVG text elements. The code also includes separate JavaScript modules for generating car reports and fetching JSON data, though these are not part of the heatmap rendering itself.