\HeaderA{update.trellis}{Retrieve and Update Trellis Object}{update.trellis}
\aliasA{t.trellis}{update.trellis}{t.trellis}
\aliasA{trellis.last.object}{update.trellis}{trellis.last.object}
\aliasA{[.trellis}{update.trellis}{[.trellis}
\keyword{dplot}{update.trellis}
\begin{Description}\relax
Update method for objects of class \code{"trellis"}, and a way to retrieve
the last printed trellis object (that was saved).
\end{Description}
\begin{Usage}
\begin{verbatim}
## S3 method for class 'trellis':
update(object,
       panel,
       aspect,
       as.table,
       between,
       key,
       auto.key,
       legend,
       layout,
       main,
       page,
       par.strip.text,
       prepanel,
       scales,
       skip,
       strip,
       strip.left,
       sub,
       xlab,
       xlim,
       ylab,
       ylim,
       par.settings,
       index.cond,
       perm.cond,
       ...)

## S3 method for class 'trellis':
t(x)

## S3 method for class 'trellis':
x[i, j, ..., drop = FALSE]

trellis.last.object(warn = TRUE, ...)
trellis.last.object()
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{object, x}] The object to be updated, of class \code{"trellis"}.

\item[\code{i, j}] indices to be used.  Names are not currently allowed. 
\item[\code{drop}] logical, whether dimensions with only one level are to be dropped.
Currently ignored, behaves as if it were \code{FALSE}.

\item[\code{warn}] logical, whether to warn when no plot is saved.

\item[\code{panel, aspect, as.table, between, key, auto.key, legend, layout,
main, page, par.strip.text, prepanel, scales, skip, strip,
strip.left, sub, xlab, xlim, ylab, ylim, par.settings, index.cond,
perm.cond, ...}] arguments that will be used to update \code{object}. See details
below. 

\end{ldescription}
\end{Arguments}
\begin{Details}\relax
All high level lattice functions such as \code{xyplot} produce an
object of (S3) class \code{"trellis"}, which is usually displayed by its
\code{print} method. However, the object itself can be manipulated and
modified to a large extent using the \code{update} method, and then
re-displayed as needed.

Most arguments to high level functions can also be supplied to the
\code{update} method as well, with some exceptions. Generally
speaking, anything that would needs to change the data within each
panel is a no-no (this includes the \code{formula, data, groups,
    subscripts} and \code{subset}). Everything else is technically game,
though might not be implemented yet. If you find something missing
that you wish to have, feel free to make a request.

Not all arguments accepted by a Lattice function are processed by
\code{update}, but the ones listed above should work.  The purpose of
these arguments are described in the help page for
\code{\LinkA{xyplot}{xyplot}}.  Any other argument is added to the list of
arguments to be passed to the \code{panel} function. Because of their
somewhat special nature, updates to objects produced by \code{cloud}
and \code{wireframe} do not work very well yet.

The \code{"["} method is a convenient shortcut for updating
\code{index.cond}.  The \code{t} method is a convenient shortcut for
updating \code{perm.cond} in the special (but frequent) case where
there are exactly two conditioning variables, when it has the effect
of switching (\sQuote{transposing}) their order.

The print method for \code{"trellis"} objects optionally saves the
object after printing it.  If this feature is on,
\code{trellis.last.object} can retrieve it.  Note that at most one
object can be saved at a time.  If \code{\LinkA{trellis.last.object}{trellis.last.object}}
is called with arguments, these are used to update the retrieved
object before returning it.
\end{Details}
\begin{Value}
An object of class \code{trellis}, by default plotted by
\code{print.trellis}.  \code{trellis.last.object} returns
\code{NULL} is no saved object is available.
\end{Value}
\begin{Author}\relax
Deepayan Sarkar \email{Deepayan.Sarkar@R-project.org}
\end{Author}
\begin{SeeAlso}\relax
\code{\LinkA{trellis.object}{trellis.object}}, \code{\LinkA{Lattice}{Lattice}},
\code{\LinkA{xyplot}{xyplot}}
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
spots <- by(sunspots, gl(235, 12, lab = 1749:1983), mean)
old.options <- lattice.options(save.object = TRUE)
xyplot(spots ~ 1749:1983, xlab = "", type = "l",
       scales = list(x = list(alternating = 2)),
       main = "Average Yearly Sunspots")
update(trellis.last.object(), aspect = "xy")
## Not run: 
trellis.last.object(xlab = "Year")
## End(Not run)
lattice.options(old.options)
\end{ExampleCode}
\end{Examples}

