Two time series of EPA groundwater monitoring data display water levels at a deep and shallow observation well. A linked brush chart below the main line chart filters the date range, and the main chart’s x-axis updates accordingly. A dashed trendline with its regression equation appears in the brushed region, while a legend identifies the two colored lines. The visualization uses d3.scaleTime, d3.line, d3.brushX, d3.nest, and a least-squares regression helper to compute the trendline from filtered data.