Given two Qini curves, \(Q_a\) and \(Q_b\), get an estimate of the difference \(Q_a(B) - Q_b(B)\), at a spend level B.
difference_gain(object.lhs, object.rhs, spend)An estimate of difference in gain along with standard errors.
A maq object \(Q_a\) to subtract from.
A maq object \(Q_b\) to subtract with.
The spend level B.