Vertical shaded reference region — a rectangle spanning
x ∈ [x1, x2] across the full plot height. Drawn behind every
data series so it never obscures the curves.
Fields:
:x1,:x2— bounds in data space (acceptDate.t()):hatch— fill pattern (default:dots_sparse):alpha— fill opacity 0..1 (default0.35):stroke— optional border dash preset;nilfor borderless:stroke_width,:label