[R] Adding error bars to a trellis barchart display

[Tim Churches]

Chris Bergstresser wrote:
> Hi all --
> 
>    I'm using trellis to generate bar charts, but there's no built-in
> function to generate error bars or confidence intervals, as far as I
> can tell.  I assumed I could just write my own panel function to add
> them, so I searched the archive, and found a posting from the author
> of the package stating "... placing multiple bars side by side needs
> specialized calculations, which are done within panel.barchart. To add
> bars to these, you will need to reproduce those calculations."
>    Just so I'm clear on this -- there's no capacity to add bars to the
> plot, nor to find out the coordinates of the bars in the graphs
> themselves.  If you want them, you have to completely rewrite
> panel.barchart.  Is this correct?  Are there really so few people
> using error bars with bar charts?

One of our projects does confidence intervals on bar cahrts produced
using teh lattice library. It is quite feasible without too much effort
- see:
http://members.optusnet.com.au/tchur/NetEpi-Analysis-0-8-Screenshot-5.png

Sorry I don't have time to extract the code which does this right now,
but you can dissect it out yourself from the NetEpi-Analysis-0.8 tarball
at http://sourceforge.net/project/showfiles.php?group_id=123700 -
although the R code is embedded in Python classes, which might make
extrication a bit more difficult (and which is why I don't have time to
do it right now). But from memory the chunk of R code which overrides
the default panel function is fairly self-contained and you should be
able to identify it fairly easily - just grep the source code for likely
strings such as "panel.barchart" to discover where it is.

Other screenshots can be downloaded from
http://sourceforge.net/project/showfiles.php?group_id=123700 if anyone
is interested.

Tim C