title: stat_contour with data labels on lines
body: <p>I wonder how to get data labels on lines in <code>ggplot2</code> for contours. Thanks</p>
  
  <pre><code>require(grDevices) # for colours
  
  x &lt;- seq(-4*pi, 4*pi, len = 27)
  y &lt;- seq(-4*pi, 4*pi, len = 27)
  r &lt;- sqrt(outer(x^2, y^2, "+"))
  
  rx &lt;- range(x &lt;- 10*1:nrow(volcano))
  ry &lt;- range(y &lt;- 10*1:ncol(volcano))
  ry &lt;- ry + c(-1, 1) * (diff(rx) - diff(ry))/2
  
  plot(
      x = 0
    , y = 0
    , type = "n"
    , xlim = rx
    , ylim = ry
    , xlab = ""
    , ylab = ""
    )
  
  contour(
      x = x
    , y = y
    , z = volcano
    , add = TRUE
    )
  
  library(ggplot2)
  library(reshape2)
  volcano3d &lt;- melt(volcano)
  names(volcano3d) &lt;- c("x", "y", "z")
  
  # Basic plot
  v &lt;- ggplot(volcano3d, aes(x, y, z = z))
  v + stat_contour()
  </code></pre>