[R] Using "|"

[Jim Milks]

Dear all,

I have a rodent dataset that I am reanalyzing.  The set consists of 
several variables (#Microtus captured, Grass stems/m^2, etc), among 
which is a Grid factor variable c(North,South).  I have evidence that 
there are significant differences in vegetation and rodent populations 
between the two locations (from chi-square tests) and would like to run 
separate analyses along North/South lines on the rest of my data.  For 
example, I type:

 > plot(Microtus.T~Grass | Grid)

However, I get the following:

Error in plot.window(xlim, ylim, log, asp, ...) :
	need finite xlim values
In addition: Warning messages:
1: "|" not meaningful for factors in: Ops.factor(Grass, Grid)
2: no finite arguments to min; returning Inf
3: no finite arguments to max; returning -Inf

Other than adding xlim arguments, what else can I do, especially in 
light of warning message 1?

Thank you in advance.

Sincerely,
Jim Milks

Graduate Student
Environmental Sciences Ph.D. Program
Wright State University
3640 Colonel Glenn Hwy
Dayton, OH 45435