R lattice package: add legend to a figure

Question

I am using the lattice package, and I want to add a legend to my figure. The documentation of auto.key and legend is very confusing, and couldn;t figure out the correct syntax to add a legend. Here is my code:

xyplot(y ~ x, df, pch=19, col=rgb(0.2, 0.4, 0.8, 0.7), cex=2,
       scales=list(cex=1.7),
       xlab=list("x", cex=1.ales=list(cex=1.7),
       xlab=list("x", cex=1.7), ylab=list("y", cex=1.7),
       main=list("Linear Regression w. Polynomial Attributes", cex=1.6),
       auto.key=T,
       panel = function(x, y, ...) {
            panel.xyplot(x, y, ...)
            llines(x, predict(lm.xtend), col="purple", lwd=6, lty=3)
            llines(x, predict(ridge.lin), col="darkgreen", lwd=6, lty=2)
       })

The graph is shown below, so I just want to add a legend for the lines.

Answer 1

I don't know exactly how you want this to look, but here is a start. In place of auto.key=T put:

key=list(space="right",
         lines=list(col=c("purple","darkgreen"), lty=c(3,2), lwd=6),
         text=list(c("Purple Line"," Dark-green Line"))
)

This will put the key on the right side of the graph. You can use "top", "bottom", or "left" instead. If you want it inside the plot, get rid of space and instead use corner=c(0,1). The first number is location on the x-axis (from 0 to 1) the second for the y-axis. So this would put in in the upper left.