title: How to add abline with lattice xyplot function?
body: <p>I am trying to plot two variables: Eeff vs Neff using xyplot function of lattice package. I would like to know how to add an line to this scatter plot (which is abline function in normal R graphic system).</p>
  
  <pre><code>xyplot(Neff ~ Eeff, data = phuong,
         xlab = "Energy efficiency (%)", 
         ylab = "Nitrogen efficiency (%)")
  </code></pre>