title: combining geom_point and geom_line in one plot
body: <p>I have two data frames similar to these:</p>
  
  <pre><code>date &lt;- c("2014-07-06", "2014-07-06","2014-07-06","2014-07-07", "2014-07-07","2014-07-07","2014-07-08","2014-07-08","2014-07-08")
  TIME &lt;- c("01:01:01", "10:02:02", "18:03:03","01:01:01", "10:02:02", "18:03:03","01:01:01", "10:02:02", "18:03:03")
  depth &lt;- c(12, 23, 4, 15, 22, 34, 22, 12, 5)
  temp &lt;- c(14, 10, 16, 13, 10, 9, 10, 14, 16)
  depth.temp &lt;- data.frame(date, TIME, depth, temp)
  depth.temp$asDate&lt;-as.Date(depth.temp$date)
  
  date &lt;- c("2014-07-06", "2014-07-07","2014-07-08") 
  meandepth &lt;- c(13, 16, 9) 
  cv &lt;- c(25, 9, 20) 
  depth.cv &lt;- data.frame(date, meandepth, cv)
  depth.cv$asDate&lt;-as.Date(depth.cv$date)
  </code></pre>
  
  <p>from the first one I have created following plot:</p>
  
  <pre><code>library(ggplot2)
  p1 &lt;- qplot(asDate, depth, data=depth.temp, colour=temp, size = I(5), alpha = I(0.3))+ scale_y_reverse()
  p1 + scale_colour_gradientn(colours = rev(rainbow(12)))
  </code></pre>
  
  <p>And from the second one this plot:</p>
  
  <pre><code>p2 &lt;- ggplot(depth.cv, aes(x=asDate, y=meandepth))+ scale_y_reverse()
  p2 + geom_line(aes(size = cv))
  </code></pre>
  
  <p>I want to merge both graphs into one with the points in the back and the line in the front, any suggestions? Note that the points and the line are NOT derived from the same data but from two different data frames.</p>