[R] Problem with data frame when using xYplot?

[Sander Oom]

Dear all,

I am trying to plot means and error bars using xYplot, but I get an 
error message from xYplot which I can not figure out:
 > Error in Summary.factor(..., na.rm = na.rm) :
         range not meaningful for factors

The data frame (tmpNa) was created using aggregate. I have used dump to 
created the code below, which generates the same error.

Can anybody tell me what is wrong with the data frame?

Thanks in advance,

Sander.

library(Hmisc)
tmpNa <-
structure(list(Position = structure(as.integer(c(1, 2, 1, 2,
1, 2, 1, 2)), .Label = c("Inside", "Outside"), class = "factor"),
     AltGeo = structure(as.integer(c(1, 1, 2, 2, 3, 3, 4, 4)), .Label = 
c("Basalt-High",
     "Basalt-Low", "Quartz-High", "Quartz-Low"), class = "factor"),
     Sodium = c(27.3333333333333, 26.8888888888889, 25, 18.1111111111111,
     4.66666666666667, 5.55555555555556, 10.6666666666667, 5.66666666666667
     ), SD = c(5.3851648071345, 2.42097317438899, 20.1618451536560,
     15.2679766541317, 5.45435605731786, 8.09492296305393, 10.6183802907976,
     8.06225774829855), Nobs = c(9, 9, 9, 9, 9, 9, 9, 9), Lower = 
c(25.5382783976218,
     26.0818978307592, 18.2793849487813, 13.0217855597339, 2.84854798089405,
     2.85724790120425, 7.12720656973412, 2.97924741723382), Upper = 
c(29.1283882690448,
     27.6958799470186, 31.7206150512187, 23.2004366624884, 6.48478535243929,
     8.25386320990686, 14.2061267635992, 8.35408591609952)), .Names = 
c("Position",
"AltGeo", "Sodium", "SD", "Nobs", "Lower", "Upper"), row.names = c("1",
"2", "3", "4", "5", "6", "7", "8"), class = "data.frame")
xYplot(Cbind(Sodium,Lower,Upper) ~ AltGeo, groups=Position,  data=tmpNa)


 > version
          _
platform i686-pc-linux-gnu
arch     i686
os       linux-gnu
system   i686, linux-gnu
status
major    2
minor    1.0
year     2005
month    04
day      18
language R

-- 
--------------------------------------------
Dr. Sander P. Oom
Animal, Plant and Environmental Sciences,
University of the Witwatersrand
Private Bag 3, Wits 2050, South Africa
Tel (work)      +27 (0)11 717 64 04
Tel (home)      +27 (0)18 297 44 51
Fax             +27 (0)18 299 24 64
Email   sander at oomvanlieshout.net
Web     www.oomvanlieshout.net/sander