This multi-line chart visualizes daily fitness trends—steps, calories burned, and average heart rate—over a 150-day period. The data is loaded from a CSV via `d3.csv` and rendered as three overlaid lines, each scaled to a common y-axis using fixed multipliers. The visualization uses D3 v7's `scaleLinear`, `axisBottom`, `axisLeft`, and `line` generators, with an SVG grid and legend, to compare the metrics' daily evolution.