A scatter plot matrix visualizes pairwise relationships among four traits of the Iris dataset (sepal length/width, petal length/width), with off-diagonal cells showing colored points by species and diagonal cells displaying histograms of each trait. Built with D3 v4, it uses `d3.scaleLinear` for axes, `d3.histogram` for binning, and `d3.brush` for interactive selection, rendering SVG with `d3.axisBottom` and `d3.axisLeft` labels.