feat(history): put synced chart values on their own curves

Value readouts on linked-but-unhovered charts now sit right on each
series' own point (a small dot plus a label at that curve's height)
instead of stacked together in one fixed corner box, so a label's
vertical position tracks its curve as the hovered time moves. The
mouse-following native tooltip is time-only everywhere, keeping every
chart's value display in the same style.

Refs #175