This visualization displays the Iris dataset as a brushable scatter plot matrix with four trait dimensions. Each cell shows a scatterplot of two traits, where circles are colored by species, while diagonal cells label the traits. Users can brush rectangular regions to filter points across all plots simultaneously. The code uses D3 v4 with an SVG renderer, implementing `d3.brush()` for interactive selection and `d3.scaleLinear`/`d3.scaleOrdinal` for axes and colors.