An interactive line chart displays blood calcium values over time for a single patient, with a vertical red line marking a selected year. Users hover over the chart to see a tooltip with the exact date and value, while a brush allows zooming into time ranges. The chart uses d3.scaleTime, d3.line, d3.brush, and d3.bisector on data loaded from a CSV file. SVG rendering and mouse events drive the interactions, with a dashed line and tooltip circle updating dynamically.