title: Drawing a boundary in on a scatterplot to identify outliers using a Copula
body: <p>I have a scatterplot around which I want to draw an elliptical boundary to identify outliers:</p>
  
  <pre><code>library(MASS)
  mu &lt;- c(7, 5); 
  sigma &lt;- matrix(c(1, 1/2, 1/2, 1), 2); 
  nu &lt;- 4
  n &lt;- 4e3 
  y &lt;- t(t(mvrnorm(n, rep(0, length(mu)), sigma) / sqrt(nu / rchisq(n, nu))) + mu)
  plot(y[,1],y[,2])
  </code></pre>
  
  <p>I can draw quantile lines:</p>
  
  <pre><code> abline(v=quantile(y[,1],c(0.05,0.95)))
   abline(h=quantile(y[,2],c(0.05,0.95)))
  </code></pre>
  
  <p>But how can I improve on this using Copulas?
  Thank you for your help.</p>