\HeaderA{interaction}{Functions to Interact with Lattice Plots}{interaction}
\aliasA{panel.identify}{interaction}{panel.identify}
\aliasA{trellis.focus}{interaction}{trellis.focus}
\aliasA{trellis.grobname}{interaction}{trellis.grobname}
\aliasA{trellis.panelArgs}{interaction}{trellis.panelArgs}
\aliasA{trellis.switchFocus}{interaction}{trellis.switchFocus}
\aliasA{trellis.unfocus}{interaction}{trellis.unfocus}
\aliasA{trellis.vpname}{interaction}{trellis.vpname}
\keyword{dplot}{interaction}
\begin{Description}\relax
The classic Trellis paradigm is to plot the whole object at once,
without the possibility of interacting with it afterwards.  However,
by keeping track of the grid viewports where the panels and strips are
drawn, it is possible to go back to them afterwards and enhance them
one panel at a time.  These functions provide convenient interfaces to
help in this.  Note that these are still experimental and the exact
details may change in future.
\end{Description}
\begin{Usage}
\begin{verbatim}
panel.identify(x, y = NULL,
               subscripts = seq_along(x),
               labels = subscripts, 
               n = length(x), offset = 0.5,
               threshold = 18, ## in points, roughly 0.25 inches
               panel.args = trellis.panelArgs(),
               ...)
trellis.vpname(name = c("position", "split", "split.location", "toplevel",
               "panel", "strip", "strip.left", "legend",
               "main", "sub", "xlab", "ylab", "page"),
               column, row,
               side = c("left", "top", "right", "bottom", "inside"),
               clip.off = FALSE, prefix)
trellis.grobname(name, prefix)
trellis.focus(name, column, row, side, clip.off,
              highlight = interactive(), ...)
trellis.switchFocus(name, side, clip.off, highlight, ...)
trellis.unfocus()
trellis.panelArgs(x, packet.number)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{x, y}] variables defining the contents of the panel.  In the
case of \code{trellis.panelArgs}, a \code{"trellis"} object. 
\item[\code{n}] the number of points to identify by default (overridden by a right
click)

\item[\code{subscripts}] an optional vector of integer indices associated with each point.
See details below.

\item[\code{labels}] an optional vector of labels associated with each point.  Defaults
to \code{subscripts}

\item[\code{offset}] the labels are printed either below, above, to the left or to the
right of the identified point, depending on the relative location of
the mouse click. The \code{offset} specifies (in "char" units) how
far from the identified point the labels should be printed.

\item[\code{threshold}] threshold in grid's \code{"points"} units. Points further than these
from the mouse click position are not considered

\item[\code{panel.args}] list that contains components names \code{x} (and usually \code{y}),
to be used if \code{x} is missing.  Typically, when called after
\code{trellis.focus}, this would appropriately be the arguments
passed to that panel.

\item[\code{name}] character string indicating which viewport or grob we are looking
for. Although these do not necessarily provide access to all
viewports and grobs created by a lattice plot, they cover most that
users might find interesting.

\code{trellis.vpname} and \code{trellis.focus} deal with viewport
names only, and only accept the values explicitly listed above.
\code{trellis.grobname} is meant to create names for grobs, and can
currently accept any value.

\item[\code{column, row}] integers, indicating position of the panel or strip that should be
assigned focus in the Trellis layout. Rows are usually calculated
from the bottom up, unless the plot was created with
\code{as.table=TRUE}

\item[\code{side}] character string, relevant only for legends (i.e. when
\code{name="legend"}), indicating their position.  Partial specification
is allowed, as long as it is unambiguous.

\item[\code{clip.off}] logical, whether clipping should be off, relevant when \code{name}
is \code{"panel"} or \code{"strip"}.  This is necessary if axes are
to be drawn outside the panel or strip.  Note that setting
\code{clip.off=FALSE} does not necessarily mean that clipping is on;
that is determined by conditions in effect during printing.


\item[\code{prefix}] character string acting as a prefix, meant to distinguish otherwise
equivalent viewports in different plots. This only becomes relevant
when a particular page is occupied by more than one plot.  Defaults
to the value appropriate for the last \code{"trellis"} object printed, as
determined by the \code{prefix} argument in
\code{\LinkA{print.trellis}{print.trellis}}.  Users should not usually need to
supply a value for this argument (see note below), however, if
supplied explicitly, this has to be a valid R symbol name (briefly,
it must start with a letter or a period followed by a letter) and
must not contain the grid path separator (currently \code{"::"})

\item[\code{highlight}] logical, whether the viewport being assigned focus should be
highlighted.  For \code{trellis.focus}, the default is \code{TRUE}
in interactive mode, and \code{trellis.switchFocus} by default
preserves the setting currently active.

\item[\code{packet.number}] integer, which panel to get data from.  See
\code{\LinkA{packet.number}{packet.number}} for details on how this is calculated

\item[\code{...}] graphical parameters. For \code{panel.identify} these are used for
labelling.  For \code{trellis.focus} and \code{trellis.switchFocus},
these are used (in combination with \code{\LinkA{lattice.options}{lattice.options}})
for highlighting the chosen viewport if so requested.

\end{ldescription}
\end{Arguments}
\begin{Details}\relax
\code{panel.identify} is similar to \code{\LinkA{identify}{identify}}.  When
called, it waits for the user to identify points (in the panel being
drawn) via mouse clicks.  Clicks other than left-clicks terminate the
procedure.  Although it is possible to call it as part of the panel
function, it is more typical to use it to identify points after
plotting the whole object, in which case a call to
\code{trellis.focus} first is necessary.

One way in which \code{panel.identify} is different from
\code{\LinkA{identify}{identify}} is in how it uses the \code{subscripts}
argument.  In general, when one identifies points in a panel, one
wants to identify the origin in the data frame used to produce the
plot, and not within that particular panel.  This information is
available to the panel function, but only in certain situations.  One
way to ensure that \code{subscripts} is available is to specify
\code{subscripts = TRUE} in the high level call such as \code{xyplot}.
If \code{subscripts} is not explicitly specified in the call to
\code{panel.identify}, but is available in \code{panel.args}, then
those values will be used.  Otherwise, they default to
\code{seq\_along(x)}.  In either case, the final return value will be
the subscripts that were marked.

The process of printing (plotting) a Trellis object builds up a grid
layout with named viewports which can then be accessed to modify the
plot further.  While full flexibility can only be obtained by using
grid functions directly, a few lattice functions are available for the
more common tasks.

\code{trellis.focus} can be used to move to a particular panel or
strip, identified by its position in the array of panels.  It can also
be used to focus on the viewport corresponding to one of the labels or
a legend, though such usage would be less useful.  The exact
viewport is determined by the \code{name} along with the other
arguments, not all of which are relevant for all names.  Note that
when more than one object is plotted on a page, \code{trellis.focus}
will always go to the plot that was created last.  For more
flexibility, use grid functions directly (see note below).

After a successful call to \code{trellis.focus}, the desired viewport
(typically panel or strip area) will be made the \sQuote{current}
viewport (plotting area), which can then be enhanced by calls to
standard lattice panel functions as well as grid functions.

It is quite common to have the layout of panels chosen when a
\code{"trellis"} object is drawn, and not before then.  Information on
the layout (specifically, how many rows and columns, and which packet
belongs in which position in this layout) is retained for the last
\code{"trellis"} object plotted, and is available through
\code{trellis.currentLayout}.

\code{trellis.unfocus} unsets the focus, and makes the top level
viewport the current viewport.

\code{trellis.switchFocus} is a convenience function to switch from
one viewport to another, while preserving the current \code{row} and
\code{column}.  Although the rows and columns only make sense for
panels and strips, they would be preserved even when the user switches
to some other viewport (where row/column is irrelevant) and then
switches back.

Once a panel or strip is in focus, \code{trellis.panelArgs} can be
used to retrieve the arguments that were available to the panel
function at that position.  In this case, it can be called without
arguments as

\begin{alltt}
    trellis.panelArgs()
\end{alltt}

This usage is also allowed when a \code{"trellis"} object is being
printed, e.g. inside the panel functions or the axis function (but not
inside the prepanel function).  \code{trellis.panelArgs} can also
retrieve the panel arguments from any \code{"trellis"} object.  Note
that for this usage, one needs to specify the \code{packet.number} (as
described under the \code{panel} entry in \code{\LinkA{xyplot}{xyplot}}) and
not the position in the layout, because a layout determines the panel
only \bold{after} the object has been printed.

It is usually not necessary to call \code{trellis.vpname} and
\code{trellis.grobname} directly.  However, they can be useful in
generating appropriate names in a portable way when using grid
functions to interact with the plots directly, as described in the
note below.
\end{Details}
\begin{Value}
\code{panel.identify} returns an integer vector containing the
subscripts of the identified points (see details above).  The
equivalent of \code{identify} with \code{pos=TRUE} is not yet
implemented, but can be considered for addition if requested.

\code{trellis.panelArgs} returns a named list of arguments that were
available to the panel function for the chosen panel.

\code{trellis.vpname} and \code{trellis.grobname} return character
strings.
\end{Value}
\begin{Note}\relax
The viewports created by lattice is accessible to the user only up to a
certain extent, as described above.  In particular,
\code{trellis.focus} can only be used to manipulate the last plot
drawn.  For full flexibility, use appropriate functions from the grid
package directly.  For example,
\code{\LinkA{current.vpTree}{current.vpTree}} can be used to
inspect the current viewport tree and
\code{\LinkA{seekViewport}{seekViewport}} or
\code{\LinkA{downViewport}{downViewport}} can be used to navigate to
these viewports.  For such usage, \code{trellis.vpname} and
\code{trellis.grobname} (with a non-default \code{prefix} argument)
provides a portable way to access the appropriate viewports and grobs
by name.
\end{Note}
\begin{Author}\relax
Deepayan Sarkar \email{Deepayan.Sarkar@R-project.org}
\end{Author}
\begin{SeeAlso}\relax
\code{\LinkA{identify}{identify}}, \code{\LinkA{Lattice}{Lattice}},
\code{\LinkA{print.trellis}{print.trellis}}, \code{\LinkA{trellis.currentLayout}{trellis.currentLayout}},
\code{\LinkA{current.vpTree}{current.vpTree}},
\code{\LinkA{viewports}{viewports}}
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}

## Not run: 
xyplot(1:10 ~ 1:10)
trellis.focus("panel", 1, 1)
panel.identify()
## End(Not run)

xyplot(Petal.Length ~ Sepal.Length | Species, iris, layout = c(2, 2))
Sys.sleep(1)

trellis.focus("panel", 1, 1)
do.call("panel.lmline", trellis.panelArgs())
Sys.sleep(0.5)
trellis.unfocus()

trellis.focus("panel", 2, 1)
do.call("panel.lmline", trellis.panelArgs())
Sys.sleep(0.5)
trellis.unfocus()

trellis.focus("panel", 1, 2)
do.call("panel.lmline", trellis.panelArgs())
Sys.sleep(0.5)
trellis.unfocus()

## choosing loess smoothing parameter

p <- xyplot(dist ~ speed, cars)

panel.loessresid <-
    function(x = panel.args$x,
             y = panel.args$y,
             span,
             panel.args = trellis.panelArgs())
{
    fm <- loess(y ~ x, span = span)
    xgrid <- do.breaks(current.panel.limits()$xlim, 50)
    ygrid <- predict(fm, newdata = data.frame(x = xgrid))
    panel.lines(xgrid, ygrid)
    pred <- predict(fm)
    ## center residuals so that they fall inside panel
    resids <- y - pred + mean(y)
    fm.resid <- loess.smooth(x, resids, span = span)
    ##panel.points(x, resids, col = 1, pch = 4)
    panel.lines(fm.resid, col = 1)
}

spans <- c(0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8)
update(p, index.cond = list(rep(1, length(spans))))
panel.locs <- trellis.currentLayout()

i <- 1

for (row in 1:nrow(panel.locs))
    for (column in 1:ncol(panel.locs))
    if (panel.locs[row, column] > 0)
{
    trellis.focus("panel", row = row, column = column,
                  highlight = FALSE)
    panel.loessresid(span = spans[i])
    grid::grid.text(paste("span = ", spans[i]),
                    x = 0.25,
                    y = 0.75,
                    default.units = "npc")
    trellis.unfocus()
    i <- i + 1
}

\end{ExampleCode}
\end{Examples}

