A brushable scatter plot matrix displays pairwise relationships among four measurements of the iris dataset, with histograms along the diagonal and colored scatter plots for each trait combination by species. Users can drag a rectangular brush across any cell to select data points, which highlights matching points across all other cells. Built with D3 v4, the visualization renders as SVG and uses `d3.brush`, `d3.scaleLinear`, `d3.scaleOrdinal` with `d3.schemeCategory10`, and `d3.histogram` for the diagonal distributions. The axes are generated via `d3.axisBottom` and `d3.axisLeft`.