\documentclass[a4paper]{article}
%\VignetteIndexEntry{Modifying multiple grobs simultaneously}
\newcommand{\grid}{{\tt grid}}
\newcommand{\R}{{\tt R}}
\setlength{\parindent}{0in}
\setlength{\parskip}{.1in}
\title{An Example of ``Linking'' Graphical Objects in Grid}
\author{Paul Murrell}

\begin{document}
\maketitle

<<echo=FALSE, results=hide>>=
library(grDevices)
library(graphics)
library(grid)
ps.options(pointsize=12)
options(width=60)

@
Suppose that I am drawing two graphs on a page, which are the results
from two subjects in an experiment.  I want the graphs to have the same
axes to aid in comparison of the subjects.  

First of all, I will split the page up into two bits for the two graphs.

<<axes1, results=hide>>=
pushViewport(viewport(layout=grid.layout(1, 2, respect=TRUE)))

@
Now I generate some data and draw the first plot.

<<axes2, results=hide>>=
x <- 1:10
y1 <- rnorm(10)
vp1a <- viewport(layout.pos.col=1)
vp1b <- viewport(width=0.6, height=0.6,
                 xscale=c(0, 11), yscale=c(-4, 4))
pushViewport(vp1a, vp1b)
grid.xaxis(name="xaxis")
grid.yaxis(name="yaxis")
grid.points(x, y1)
popViewport(2)

@
Notice that I have named the graphical objects for the axes.
When I draw the second plot I will use the same names for the
axis objects.

<<axes3, results=hide>>=
y2 <- rnorm(10)
vp2a <- viewport(layout.pos.col=2)
vp2b <- viewport(width=0.6, height=0.6,
                 xscale=c(0, 11), yscale=c(-4, 4))
pushViewport(vp2a, vp2b)
grid.xaxis
grid.xaxis(name="xaxis")
grid.yaxis(name="yaxis")
grid.points(x, y2)
popViewport(2)

@
The output looks like the figure below.

<<shared, results=hide, fig=TRUE, echo=FALSE, width=4, height=2, include=FALSE>>=
<<axes1>>
<<axes2>>
<<axes3>>

@
\begin{center}
\includegraphics[width=4in, height=2in]{sharing-shared}
\end{center}

@

Because I have used the same name for the axis objects in both plots,
 I can edit the axes for both plots 
simultaneously rather than having to edit each one in turn.
For example ...

<<axesedit, results=hide>>=
grid.edit("xaxis", at=c(1, 5, 9), global=TRUE)

@
The output now looks like the figure below.

<<shared2, results=hide, fig=TRUE, echo=FALSE, width=4, height=2, include=FALSE>>=
postscript("shared2-%02d.eps", onefile=FALSE, paper="special",
  width=4, height=2)
<<axes1>>
<<axes2>>
<<axes3>>
<<axesedit>>

pdf("shared2-%02d.pdf", onefile=FALSE,
  width=4, height=2)
<<axes1>>
<<axes2>>
<<axes3>>
<<axesedit>>

@
\begin{center}
\includegraphics[width=4in, height=2in]{shared2-02}
\end{center}

@

This might seem a very small gain in this example, but it is
potentially of great use in, for example, the implementation of a scatterplot
matrix.

@
\end{document}