title: stack contour plot in R
body: <p>I have two different matrices (same x and y axis) which a plot as contour plots contour (x,y,z). I would like to figure out how to stack both matrix into a single plot in R.</p>
  
  <p>I've been trying to use the as.layer function but it doesn't work</p>
  
  <pre><code>heteroplot &lt;- contour(a[,1],t(a[,1]),nlevels=7,heterocov^2,col="green",xlab="ppm",ylab="ppm",bty="n")
  
  homocov &lt;- contour(a[,1],t(a[,1]),nlevels=7,cova^2,col="red",xlab="ppm",ylab="ppm",bty="n")
  
  as.layer(homocov,x.same = TRUE, y.same = TRUE)
  </code></pre>
  
  <p>thanks!</p>