This area chart visualizes the distribution of diagnoses by count, using a smooth monotone curve to connect points across categorical diagnosis labels. The chart is rendered as an SVG via D3 v7, with an area path filled in steel blue beneath the line. The code groups the source CSV data with `d3.rollups` to count occurrences per diagnosis, then scales the x-axis with `d3.scalePoint` and the y-axis with `d3.scaleLinear`. Axes are generated using `d3.axisBottom` and `d3.axisLeft`.