title: Plot 95% confidence limits in scatterplot
body: <p>I need to plot several data points that are defined as </p>
  
  <blockquote>
    <p>c(x,y, stdev_x, stdev_y)</p>
  </blockquote>
  
  <p>as a scatter plot with a representation of their 95% confidence limits, for examples showing the point and one contour around it. Ideally I'd like to plot on oval around the point, but don't know how to do it. I was thinking of building samples and plotting them, adding stat_density2d() but would need to limit the number of contours to 1, and could not figure out how to do it. </p>
  
  <pre><code>require(ggplot2)
  n=10000
  d &lt;- data.frame(id=rep("A", n),
                  se=rnorm(n, 0.18,0.02), 
                  sp=rnorm(n, 0.79,0.06) )
  g &lt;- ggplot (d, aes(se,sp)) +
    scale_x_continuous(limits=c(0,1))+
    scale_y_continuous(limits=c(0,1)) +
    theme(aspect.ratio=0.6)
  g + geom_point(alpha=I(1/50)) +
    stat_density2d()
  </code></pre>