Detrend Non-linear Data

Removes nonlinear trend from data

DETRENDNONLIN Removes non-linear trend from a vector.
The nonlinearity is removed by subtracting a least-squares
polynomial fit.
Y = DETRENDNONLIN(X) subtracts a second order polynomial fit from
the data vector X.
Y = DETRENDNONLIN(X, n) subtracts a polynomial fit of order n from
the data vector X.
Note: This function may throw errors generated in the function call to
POLYFIT.