title: Adding a third axis to a scatter plot in ggplot2
body: <p>I just have a question about how to add a third axis(a secondary x axis) to a existing scatter plot. 
  For example, I have the following dataframe: </p>
  
  <pre><code>df = data.frame(a = c(10,20,30,40,50,60,70),b = c(12,15,21,21,22,26,31), c = c(0.1,0.2,0.3,0.3,0.3,0.4,0.5))
  </code></pre>
  
  <p>I am using "a" as the x-axis and "b" as the y-axis and plot a scatter plot. Is that possible that I have a secondary x-axis on the top of the panel to indicate the corresponding "c" value? </p>