This interactive D3 v5 visualization compares groundwater levels at two EPA monitoring wells against effective rainfall in a swamp marsh, using a main line chart with a linked brush chart and vertical bar histogram. Users brush a time range in the lower chart to filter the main view, which recomputes a dashed trendline via a least-squares regression function and displays its equation. The upper SVG rendering includes dual y-axes, a categorical color legend, and derives data from external CSV files parsed with `d3.timeParse` and plotted with `d3.line`, `d3.brushX`, and `d3.axisBottom`.