Four heatmaps of 100 household incomes in the US are drawn side-by-side using D3.js (v6) SVG rects. Each heatmap applies a different color scale: linear, quantize, quantile, and threshold. The linear scale maps the data extent to a white-to-red gradient, quantize creates three uniform bins, quantile creates equal-count bins, and threshold uses custom domain breaks. The code uses `d3.scaleLinear`, `d3.scaleQuantize`, `d3.scaleQuantile`, and `d3.scaleThreshold`.