By default, uses linear interpolation for non-seasonal series. For seasonal series, a robust STL decomposition is first computed. Then a linear interpolation is applied to the seasonally adjusted data, and the seasonal component is added back.
Box-Cox transformation parameter. If
then a transformation is automatically selected using
The transformation is ignored if NULL. Otherwise,
data transformed before model is estimated.
Should a linear interpolation be used.
A more general and flexible approach is available using