Build a SweatStack app showing a heart rate histogram for running activities, comparing the last 7 days against a 42-day baseline. Overlay both datasets, normalizing 42-day counts to 7-day equivalent. Allow changing the reference date.

Styling: 42-day bars transparent with border, 7-day bars with diagonal hatching. Hovering a legend item dims the other dataset. Bar tooltips show bin range and % difference vs baseline. Grayscale.