R help - May 2006 - Adding error bars to a trellis barchart display

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?

-- Chris

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

On 5/13/06, Chris Bergstresser <chris at subtlety.com>
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?
I would hope so, since bar charts with error bars are horrible things,
in my opinion. Lattice comes with an example of (standard) confidence
intervals (new in R 2.3.0) that can be accessed by running

library(lattice)
demo(intervals)

The Hmisc package also has high level interfaces that support
confidence intervals (I'm not familiar with the details).

-Deepayan