How to control the dimension / size of a plot with ggplot2

Question

I am using ggplot2 (respectively qplot) to generate a report with Sweave. Now I need some help with the adjustment of the size of the plot. I use the following Sweave code to include it.

\begin{figure}[htbp]
\begin{center}
<<fig=true,echo=false>>=
print(mygraph)
@
\caption{MyCaption}
\end{center}
\end{figure}

If I add a width argument (like shown below) to plot is squeezed down, but not really scaled down.

<<fig=true,echo=false,width=3>>=

If I use ggsave() instead, I could use a scale argument and influence the size of the resulting .pdf file. Is there a way to influence the dimensions of the plot without saving it (since the .pdf is generated by Sweave anyway) ? Is there anything I need to add to my qplot code?

mygraph=qplot(date,value,data=graph1,geom="line",colour=variable,xlab="",ylab="")
+ scale_y_continuous(limits = c(-0.3,0.3))

Thx for any suggestions in advance!

Answer 1

Instead of doing this within ggplot2, add the following LaTeX code before the code chunk where you print the graph.

\SweaveOpts{width=x, height=y}

x and y are height and width in inches.

If there is a particular aspect ratio you would like your plot to be, you can set this in ggplot2 with opts(). Unless I have some other reason, I usually try to keep my plots scaled to the golden ratio, per Tufte's suggestions. Usually I have

...
SweaveOpts{width=8, height=5}
...
<<label = "makeplot", echo = F>>=
  p <- ggplot(mpg, aes(displ, hwy)) + 
    geom_point()+
    opts(aspect.ratio = 2/(1+sqrt(5)) )
@
...
\begin{figure}[htbp]
\begin{center}
<<fig=true,echo=false>>=
  print(p)
@
\caption{MyCaption}
\end{center}
\end{figure}

Answer 2

The Sweave options width and height influence the dimensions of the PDF file but not the size of the figures in the document. Put something like

\setkeys{Gin}{width=0.4\textwidth}

after \begin{document} to get smaller plots.

Source: Sweave manual, sec. 4.1.2