\HeaderA{identify}{Identify Points in a Scatter Plot}{identify}
\methaliasA{identify.default}{identify}{identify.default}
\keyword{iplot}{identify}
\begin{Description}\relax
\code{identify} reads the position of the graphics pointer when the
(first) mouse button is pressed.  It then searches the coordinates
given in \code{x} and \code{y} for the point closest to the pointer.
If this point is close enough to the pointer, its index will be returned as
part of the value of the call.
\end{Description}
\begin{Usage}
\begin{verbatim}
identify(x, ...)

## Default S3 method:
identify(x, y = NULL, labels = seq(along = x), pos = FALSE,
         n = length(x), plot = TRUE, atpen = FALSE, offset = 0.5,
         tolerance = 0.25, ...)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{x,y}] coordinates of points in a scatter plot.  Alternatively, any
object which defines coordinates (a plotting structure, time
series etc: see \code{\LinkA{xy.coords}{xy.coords}}) can be given as \code{x},
and \code{y} left undefined.
\item[\code{labels}] an optional character vector, the same length as \code{x} and
\code{y}, giving labels for the points.  Will be coerced using
\code{\LinkA{as.character}{as.character}}.
\item[\code{pos}] if \code{pos} is \code{TRUE}, a component is added to the
return value which indicates where text was plotted relative to each
identified point: see Value.
\item[\code{n}] the maximum number of points to be identified.
\item[\code{plot}] logical: if \code{plot} is \code{TRUE}, the labels are printed at
the points and if \code{FALSE} they are omitted.
\item[\code{atpen}] logical: if \code{TRUE} and \code{plot = TRUE}, the
lower-left corners of the labels are plotted at the points clicked
rather than relative to the points.
\item[\code{offset}] the distance (in character widths) which separates the
label from identified points.  Ignored if \code{atpen = TRUE} or
\code{pos = 0}.
\item[\code{tolerance}] the maximal distance (in inches) for the pointer to be
\sQuote{close enough} to a point.
\item[\code{...}] further arguments passed to \code{\LinkA{par}{par}} such as
\code{cex}, \code{col} and \code{font}.
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
\code{identify} is only supported on screen devices such as
\code{X11}, \code{windows} and \code{quartz}.  On other devices the
call will do nothing.

If \code{plot} is \code{TRUE}, the point is labelled with
the corresponding element of \code{text}.
The labels are placed below, to the left, above or to the right
of the identified point, depending on where the cursor was relative to
the point.

For the usual \code{\LinkA{X11}{X11}} device the identification process is
terminated by pressing any mouse button other than the first.
For the \code{\LinkA{quartz}{quartz}} device the process is terminated by
pressing the \code{ESC} key.

On most devices which support \code{identify}, successful selection of
a point is indicated by a bell sound unless
\code{\LinkA{options}{options}(locatorBell = FALSE)} has been set.

If the window is resized or hidden and then exposed before the identification
process has terminated, any labels drawn by \code{identify}
will disappear.  These will reappear once the identification process has 
terminated and the window is resized or hidden and exposed again.
This is because the labels drawn by \code{identify} are not
recorded in the device's display list until the identification process has
terminated.
\end{Details}
\begin{Value}
If \code{pos} is \code{FALSE}, an integer
vector containing the indexes of the identified points.

If \code{pos} is \code{TRUE}, a list containing a component
\code{ind}, indicating which points were identified and a component
\code{pos}, indicating where the labels were placed relative to the
identified points (1=below, 2=left, 3=above, 4=right and 0=no offset,
used if \code{atpen = TRUE}).
\end{Value}
\begin{References}\relax
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
\emph{The New S Language}.
Wadsworth \& Brooks/Cole.
\end{References}
\begin{SeeAlso}\relax
\code{\LinkA{locator}{locator}}
\end{SeeAlso}

