Hover a chart to read the values at that instant

[evandhoffman]

A chart shows what happened; it does not let you ask what happened then. The legend carries the latest value of every series, so the moment you want a number from is the only moment you cannot read.

Hover anywhere on a chart and get the values at that point in time.

Sketch

• A vertical rule follows the pointer, snapped to the nearest sample rather than to the pixel — there is a sample every half second and nothing exists between them.
• Every series on the card reports its value at that instant, including a stacked card's bands and a mirrored card's two directions.
• Move off the chart and it returns to the latest values.

Open question — where do the numbers appear?

1. In the legend that is already there. The header reserves a fixed-width slot per series for exactly this kind of number (see "A legend that does not shuffle" in docs/ui.md), so hovering could swap "current" for "at the cursor" and add no chrome at all. The time would need somewhere to go — probably the title's line.
2. A floating tooltip beside the pointer. The conventional answer. It covers the trace it is describing, and on a 260 pt card there is not much room to be covered.

Option 1 looks better here and is cheaper. Worth deciding before any code.

Notes

• A mirrored card must report the magnitude, not the plotted value. Below the baseline is a direction, not a negative quantity — same rule the axis labels follow.
• A stacked card should report each band's own value, not its cumulative height. The height is an artefact of the drawing; the value is the metric.
• The tile already carries a left-drag (reorder) and a right-click (copy). A hover must not interfere with either, and the rule must not appear while a drag is in flight.
• chartOverlay with a DragGesture of minimumDistance: 0 is the usual route, which is exactly the gesture the reorder drag uses. Check the two do not fight, in swift run monitor and not only in tests.
• Nothing here should touch what is stored. It is view state, like the zoom in Double-click a card to zoom it #29.