The transform vector can be NULL for none.
Otherwise xv is a vector that specifies a sequence of
transform functions.Example:
xv = c("log","season",12,"trend",1)
The above tansform vector
takes the log of the data, then subtracts a seasonal component
of period 12, then subtracts a linear trend component.
There are five functions from which to choose.
ll{
diff Difference the data. Has a single argument, the lag.
hr Subtract harmonic components. Has one or more
arguments, each specifying the number of observations per harmonic.
log Take the log of the data, has no arguments.
season Subtract a seasonal component.
Has a single argument, the number of observations per season.
trend Subtract a trend component.
Has a single argument, the order of the trend
(1 linear, 2 quadratic, etc.)
}
At the end of the transform vector there is an implied subtraction of
the mean operation.
Hence the resulting time series always has zero mean.