Add abline to filled contour plot

Question

I drew a contour plot and added a diagonal line:

library(MASS)
library(fields)
x <- c(21.06, 28.89, 23.00, 23.61, 23.61, 22.83, 30.44)
y <- c(26.56, 24.00, 13.06, 18.61, 18.61, 14.17, 25.33)
z <- kde2d(x, y, n=32, lims = c(0,32,0,32))
contour(z, col = "red", main = "Density estimation: contour plot",
       las=0,

    plot.title=
               {
      title(xlab=expression(alpha),cex.lab=2)
      mtext(expression(beta),2,cex=2,line=3,las=1)
               } 
)
abline(0, 1, col = "red", lwd = 2) 

I think I like the filled contour more, but now the line is off:

filled.contour(z,  plot.title={
   title(main = "Density estimation: contour plot")
   title(xlab=expression(alpha),cex.lab=2)
   mtext(expression(beta),2,cex=2,line=3,las=1)
 })
 abline(0, 1, col = "red", lwd = 2) 

Answer 1

You could throw the abline in there:

library(MASS)

x <- c(21.06, 28.89, 23.00, 23.61, 23.61, 22.83, 30.44)
y <- c(26.56, 24.00, 13.06, 18.61, 18.61, 14.17, 25.33)
z <- kde2d(x, y, n = 32, lims = c(0, 32, 0, 32))

filled.contour(z,  plot.title={
  title(main = "Density estimation: contour plot")
  title(xlab=expression(alpha),cex.lab=2)
  mtext(expression(beta),2,cex=2,line=3,las=1)
  abline(0, 1, col = "red", lwd = 2)
})

Or (as was suggested in a comment), you might be better off doing something like:

filled.contour(
  z,
  plot.title = {
    title(main = "Density estimation: contour plot")
    title(xlab = expression(alpha), cex.lab = 2)
    mtext(expression(beta), 2, cex = 2, line = 3, las = 1)
  },
  plot.axes = {
    abline(0, 1, col = "red", lwd = 2)
  }
)

Simply because it feels a little weird putting an abline in plot.title.