[R] ltext, plotmath, and substitute

Dave Atkins datkins at u.washington.edu
Wed Apr 14 18:13:39 CEST 2004

I am interested to use plotmath functions within a panel function but am having 
some problems getting the code right.  Within each panel I am plotting the data, 
fitting a regression line, and would like to print the regression equation. 
Here is a trivial example of what I'd like to do:

# generate simple data
tmp.df <- data.frame(id = rep(1:4, each=4),
			time = rep(1:4, 4),
			das = rnorm(16))
# plot regression lines and print equations
xyplot(das ~ time | as.factor(id), data = tmp.df, strip = T,
	   panel = function(x,y,...){
		 panel.coef <- round(coef(lm(y~x)), 2)
		 ltext(1.2, 1, pos = 4,
		   paste("Y = ",panel.coef[1]," + ",panel.coef[2],"*time",
			sep = ""), cex = 1.2)
	   layout = c(4,1,1), aspect = 2.5)

The above works fine.  However, I'd like to make the equations more attractive 
by adding a hat over Y and an epsilon at the end, and this is where I have run 
into problems.  I experimented with expression() and paste() for a while without 
luck, and now believe I need to use substitute() (given some of the examples on 
the plotmath() help-page).  But, I can't get the code quite right.

Any suggestions appreciated.

Dave Atkins
datkins at u.washington.edu

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