\Header{locator}{Graphical Input}
\keyword{iplot}{locator}
\begin{Description}\relax
Reads the position of the graphics cursor when the (first) mouse
button is pressed.
\end{Description}
\begin{Usage}
\begin{verbatim}
locator(n = 512, type = "n", ...)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{n}] the maximum number of points to locate.  Valid values start
at 1.
\item[\code{type}] One of \code{"n"}, \code{"p"}, \code{"l"} or
\code{"o"}. If \code{"p"} or \code{"o"} the points are
plotted; if \code{"l"}  or \code{"o"} they are joined by lines.
\item[\code{...}] additional graphics parameters used if \code{type != "n"}
for plotting the locations.
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
\code{locator} is only supported on screen devices such as
\code{X11}, \code{windows} and \code{quartz}.  On other devices the
call will do nothing.

Unless the process is terminated prematurely by the user (see below)
at most \code{n} positions are determined.

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

The current graphics parameters apply just as if \code{plot.default}
has been called with the same value of \code{type}. The plotting of
the points and lines is subject to clipping, but locations outside the
current clipping rectangle will be returned.

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

If the window is resized or hidden and then exposed before the input
process has terminated, any lines or points drawn by \code{locator}
will disappear.  These will reappear once the input process has
terminated and the window is resized or hidden and exposed again.
This is because the points and lines drawn by \code{locator} are not
recorded in the device's display list until the input process has
terminated.
\end{Details}
\begin{Value}
A list containing \code{x} and \code{y} components which are the
coordinates of the identified points in the user coordinate system,
i.e., the one specified by \code{\Link{par}("usr")}.
\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{\Link{identify}}
\end{SeeAlso}

