This heatmap visualizes hourly activity across the days of the week, using color intensity to show higher counts in steel blue against white. The example loads a CSV dataset with d3.csv, then maps day names to an ordinal x-scale and hours to a linear y-scale. A second linear scale maps data values to the fill color of SVG rectangles, which are rendered as a 2D histogram. The code uses d3.scale.ordinal, d3.scale.linear, and d3.max to compute domains, though the tile-drawing and axis code is commented out, leaving only the scales and data loading active.