Two interactive line charts display monthly water level measurements for two wells, sharing a synchronized brush. The upper chart shows the full dataset, while the lower chart's brush selection filters the time domain of the upper chart via `d3.brushX` and `event.selection`. Both use `d3.line`, `d3.nest`, and `d3.scaleTime` to render multiple colored series per well, with data loaded from a CSV. The `brushChart` function is instantiated twice in `index.js`, demonstrating reusable chart components.