This area chart visualizes the number of iPhone users from 2007 to 2017, using a smooth cubic basis curve for both the line and the filled region beneath it. The chart is rendered as an SVG, with linear scales for both the year (x-axis) and user count (y-axis). The code leverages D3 v5's `select`, `csv`, `scaleLinear`, `line`, and `area` APIs to load data from a CSV file, format the y-axis tick labels with `format('.3s')`, and append axis labels and grid lines.