CurvFit ™ is a nonlinear curve fitting program. Sine, damped Sine, Lorentz, Modified Lorentz, Power (ie Polynomial) and Exponential series are presently available models to match your data. We strongly suggest trying a Lorentz series for data with multiple peaks or valleys. A calculator exists for interpolation &/or extrapolation of given data. CurvFit has proven excellent for hard to fit data. Hard to fit data may take more time -but- it can be done given the right series and parameter values. For start try curve fitting your data with a Lorentz series!
A Lorentz function equals 1 / (1 + a x2). This is a shortened form of the infinite series inverse (1 + Σ ai x2i). For practical purposes the shortened Lorentz function is accurate enough. The Lorentz function equals the derivative of the arctangent.
A Modified Lorentz function equals (1 + x) / (1 + a x2) = (1 + x) * Lorentz function! Use the modified Lorentz when minimizing number of terms in your curve fit series. (Someone suggested that the modified Lorentz is a Bessel function, is it?)