This example displays a correlation matrix as a square grid of colored cells, using a diverging red-white-navy color scale to represent values from -1 to 1. The dataset is a correlation matrix of automobile metrics (from the classic `mtcars` data), with each cell showing a correlation coefficient or variable name. The lower triangle uses text values, while the upper triangle renders circles whose radius scales with the absolute correlation. Built with D3 v3 and SVG, it includes a custom color legend axis generated with `d3.svg.axis` and a series of colored rectangles.