This line chart visualizes the historical Bitcoin Price Index from December 2017 to April 2018, using data fetched from the CoinDesk API. The SVG chart displays price over time with a steelblue line, axis labels, and a rotated y-axis title. The example demonstrates D3.js v4's `d3.scaleTime`, `d3.scaleLinear`, `d3.line`, and `d3.axisBottom`/`d3.axisLeft` for building a complete, responsive chart from JSON data parsed into date-value pairs.