\item{x2}{a numeric vector giving the coordinates of the observations
on the \emph{y}-axis. Both \code{x1} and \code{x2} must be specified
to be used.}
\item{x2}{a numeric vector giving the coordinates of the observations
on the \emph{y}-axis. Both \code{x1} and \code{x2} must be specified
to be used.}