Plot decision boundaries with ggplot2?

Question

How do I plot the equivalent of contour (base R) with ggplot2? Below is an example with linear discriminant function analysis:

require(MASS)
iri         


        

Answer 1

You can use geom_contour in ggplot to achieve a similar effect. As you correctly assumed, you do have to transform your data. I ended up just doing

pr<-data.frame(x=rep(x, length(y)), y=rep(y, each=length(x)), 
    z1=as.vector(iris.pr1), z2=as.vector(iris.pr2))

And then you can pass that data.frame to the geom_contour and specify you want the breaks at 0.5 with

ggplot(datPred, aes(x=LD1, y=LD2) ) + 
    geom_point(size = 3, aes(pch = Species,  col=Species)) + 
    geom_contour(data=pr, aes(x=x, y=y, z=z1), breaks=c(0,.5)) + 
    geom_contour(data=pr, aes(x=x, y=y, z=z2), breaks=c(0,.5))

and that gives