A single-series area chart shows the distribution of patient diagnoses from a heart disease dataset, with the curve encoding diagnosis categories on the x-axis and case counts on the y-axis. The chart uses a monotone curve to smooth transitions between points, with a steelblue filled area beneath the line. The code loads CSV data via `d3.csv`, aggregates counts with `d3.rollups`, and renders the SVG using `d3.area`, `d3.scalePoint`, `d3.scaleLinear`, and `d3.axisBottom`/`d3.axisLeft`.