Lattice graphics adding abline line (1:1 line) ???

I have read multiple books and looked at many posts online and can't seem to
figure out how to add a 1:1 line in lattice xyplot. I've tried multiple
versions of getting this to work, including trying "panel.abline(h=0,v=1),
panel=function and others.

Second question, how do I get the legend created via "auto.key" to use the
"pch=c(15,16,17,18) and associated colors, these are circle, square,triangle
and diamond, but the auto.key just plots points with different default
colors, not the grey scales and symbols that the actual plots uses, how do I
get these to match. Is auto.key not the way to go, again I read on many
different methods, but no luck yet.

Thanks,
     Ian

xyplot(Inv591$RichTOL~gbm_tol$fitted, data=Inv591,group=EcoRegion, main=
"Observed vs Predicted for RichTOL (BRT development model)",xlab="Observed
RichTOL", ylab="Predicted RichTOL", xlim= c(2,8), ylim= c(2,8),
pch=c(15,16,17,18), cex=1.1,
col=c('grey10','grey30','grey60','grey80'),cex.lab=1.8, cex.axis=1.8,font=2,
auto.key=list(corner=c(1,0)),
panel=function(x,y){
panel.xyplot(x,y)
panel.abline(lm(Inv591$RichTOL~gbm_tol$fitted))

I don't know what a "1:1 line" is, but your (nonreproducible)
code suggests that you want a regression line (or lines?).
As for auto.key, it may be better to use the "key" argument.

Here are a couple of suggestions (using the iris data)
that may help you:

1. If you just want a single regression line for all groups
combined:

   library(lattice)
   xyplot(Sepal.Length ~ Sepal.Width, data = iris,
     groups = Species,
     pch = 15:17, col = 2:4,
     panel = function(x,y,...){
       panel.xyplot(x,y,...)
       panel.lmline(x,y)},
     key = list(corner = c(1,0),
             text = list(lab = levels(iris[["Species"]])),
             points = list(pch = 15:17, col = 2:4)))

2. If you want separate regression lines for each group:

   xyplot(Sepal.Length ~ Sepal.Width, data=iris,
     groups = Species,
     pch = 15:17, col = 2:4,
     panel = "panel.superpose",
     panel.groups = function(x,y,...){
       panel.xyplot(x,y,...)
       panel.lmline(x,y,...)
     },
     key = list(corner = c(1,0),
             text = list(lab = levels(iris[["Species"]])),
             points = list(pch = 15:17, col = 2:4)))

If you use panel.abline for regression lines, be sure to
specify the regression formula with the "reg=" argument.

Peter Ehlers