title: R saving contour lines into a dataframe
body: <p>ggplot allows me to plot contour lines for a data series x and y:</p>
  
  <pre><code>library(ggplot2)
  df &lt;- data.frame(x=c(1:100),y=rnorm(100))
  ggplot(df,aes(x=x,y=y))+geom_density2d()
  </code></pre>
  
  <p>I would like to be able to save the data output of the main four contour lines (95,75,50,25)max and (95,75,50,25)min in a  dataframe.
  I would be grateful for your help.
  Perhaps there is a way of doing this directly, without using ggplot.</p>