This example demonstrates an interactive area chart built with D3.js (v4) that presents two overlaid series with gradient fills, rendered as SVG. The visualization initially displays one dataset, and a button click transitions the chart to a second, extended dataset. The chart's axes and area paths smoothly animate their transitions over two seconds using D3's `.transition()` method. The code defines linear gradients with specified opacity stops for series styling, and uses `d3.scaleLinear` and `d3.axisBottom`/`d3.axisRight` for scaling and axes, respectively.