[R] Plotting Factorial GLMs

[Jarrett Byrnes]

Hello all,
	I'm attempting to plot the functions from a generalized linear model 
while iterating over multiple levels of a factor in the model.  In 
other words, I have a data set

Block, Treatment.Level, Response.Level

So, the glm and code to plot should be

logit.reg<-glm(formula = Response.Level ~ Treatment.Level + Block,
			family=quasibinomial(link="logit")))

plot( Response.Level ~ Treatment.Level)

logit.reg.function <- function (trt, blk) predict(logit.reg, 
data.frame(Treatment.Level=trt, Block=blk)

curve(logit.reg.function(x, "A"), add=TRUE)


But I get the error:
Error in xy.coords(x, y) : 'x' and 'y' lengths differ

Now, if I set Block="A" in the function, and take blk out, as well as 
taking the "A" out of the curve statement, it plots just fine.  What am 
I doing wrong, as this would be a nice, quick, and easy way to whip up 
multiple curves from a factorial dataset!