The scale's domain that specifies the input values.
@override get domain { return [_x0, _x1]; }
@override set domain(domain) { _x0 = domain[0]; _x1 = domain[1]; _t0 = _transform(_x0); _t1 = _transform(_x1); _k10 = _t0 == _t1 ? 0 : 1 / (_t1 - _t0); }