Fill between two curves sharing x (type: 'band') — confidence intervals,
min–max envelopes, and threshold fills. Not area-to-baseline and not annotation bandX.
Static mode: ±1σ confidence band around a noisy mean series.
Live mode: streaming min/max envelope via appendData with FIFO maxPoints.
Hover reports y and y1.
Live tip: wheel/pan pauses auto-scroll so you can zoom back out freely.
Check Auto-scroll again to re-follow the newest samples.