The reference quantity v is linearly interpolated in piecewise fashion according to the QuantLib Linear method.
Formally, given any two successive nodes (x₁,v₁) and (x₂,v₂) then the value v corresponding to some x lying between x₁ and x₂ is given by
v = w₁*v₁ + w₂*v₂
where the weights are given by
w₁ = (x₂-x)/(x₂-x₁)
w₂ = 1 - w₁