This line chart compares the frequency of earthquakes across four U.S. regions from 2011 to 2015, using data from a CSV file that includes state, region, year, and count. The visualization uses D3 v5 with `nest()` and `rollup()` to aggregate counts by region and year, then draws smoothed lines with `line()` and `curveBasis` to show trends. Color-coded circles mark yearly data points, and a custom `colorLegend` function displays the regional color key. The chart is rendered as SVG with axes from `axisLeft` and `axisBottom`, and includes interactive hover effects on the points.