mergeOver method
This style with nulls filled from base (per-point over series default).
Color and gradient stay mutually exclusive: a point that sets color suppresses the base's gradient, and vice versa.
Implementation
BarStyle mergeOver(BarStyle base) => BarStyle(
color: color ?? (gradient == null ? base.color : null),
gradient: gradient ?? (color == null ? base.gradient : null),
borderRadius: borderRadius ?? base.borderRadius,
border: border ?? base.border,
width: width ?? base.width,
);