A 4x4 scatterplot matrix visualizes the classic Iris dataset, with each cell plotting one pair of four flower measurements and the diagonal showing histograms of each feature. Points are colored by species using `d3.scaleOrdinal`. Rendering in SVG via D3 v6, the visualization incorporates `d3.brush` with a callback that filters the original data to highlight points within the brushed selection across all plots. Binning for the histograms uses `d3.histogram`, with axes generated by `d3.axisBottom` and `d3.axisLeft`.