This visualization shows monthly water level trends for two well sites, with a brushable chart allowing users to select a date range and view a dashed regression trendline with its equation. The top chart displays the trendline and data points, while the bottom chart provides the brushing interface. The code uses d3.scaleTime, d3.scaleLinear, d3.line, d3.brushX, and d3.nest to render SVG paths and circles. The trendline is calculated via a custom least squares regression function imported from a separate module. Data is loaded from a CSV file.