Alias for x0, but additionally sets x1 to null when defined.
final area = Area(…)..x = (d, i, data) => x(d["Date"]); area.x; // (d, i, data) => x(d["Date"])
num Function(T, int, List<T>) get x => _x0;
set x(num Function(T, int, List<T>) x) { _x0 = x; _x1 = null; }