Re: [R] Adding error bars to a trellis barchart display

From: Tim Churches <tchur_at_optushome.com.au>
Date: Sun 14 May 2006 - 10:19:15 EST

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



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