title: Invert colored area geom_area
body: <p>I am sure it is easy to solve this question.</p>
  
  <p>I am trying to fill the area below the line I plot from "mydata" using <code>ggplot2</code>.</p>
  
  <p>here the example:</p>
  
  <pre><code>a = seq(10, 100, 5)
  b = -(runif(19, 0, 50))
  mydata &lt;- data.frame(a, b)
  
  ggplot(data = mydata, aes(x = a, y = b)) +
    geom_area()
  </code></pre>
  
  <p><code>geom_bar</code> fills above the line, considering the area the one from 0 and the values (negative). I want to fill the other area and I think I should delimit the area I want to colour, but I do not know how.</p>
  
  <p>The script above is just an easy example. 
  My real script is </p>
  
  <pre><code>ggplot(data = ctd, aes(x = Longitude, y = Depth)) +
    geom_raster(aes(fill = NO3_uM)) +
    scale_fill_gradientn(colours = rev(my_colours)) +
    geom_contour(aes(z = NO3_uM), binwidth = 2, colour = "black", alpha = 0.2) +
    #geom_contour(aes(z = NO3_uM), breaks = 20, colour = "black") +
  
    geom_point(data = ctd, aes(x = Longitude, y = Depth),
               colour = 'black', size = 3, alpha = 1, shape = 15) +
    geom_area(data = trsect, aes(x = Longitude, y = Depth), fill = "black")+
    ylim(-320,0)
  </code></pre>
  
  <p>my result is:</p>
  
  <p><a href="https://i.stack.imgur.com/dwGCD.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/dwGCD.png" alt="enter image description here"></a>
  <a href="https://i.stack.imgur.com/EJRpy.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/EJRpy.png" alt="enter image description here"></a></p>
  
  <p>I want to colour the other area of the "mountain" </p>