[R] Why don't formulas that work for lm() work for plotmeans?

[jrflanders]

I know that simple line plots showing a point representing the mean + an
error bar are difficult in R, but I  am clearly missing something.  I simply
want to show how a the mean and 95% CI of a chemical concentration (y) vary
over space (x) under four temperature regimes (T; a categorical factor).
plotmeans, in the gplots library, should allow for such a figure, right?
After all, it invokes lm() in the help file for the formula, so any formula
that works in lm() (e.g., y~x | T) should work in plotmeans.  Obviously this
is not so - can anyone suggest an alternative  command or help me understand
how  the syntac differs? Thanks...
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