title: Shading an area between geom_line and the x-axis
body: <p>I have two plots displaying supply and demand, and one plot in which I have subtracted the demand from the supply to show the resulting asymmetry. I would like to shade the area between the x-axis and the negative part of the asymmetry, to show the extent of the deficit. </p>
  
  <p>I currently use the following code:</p>
  
  <pre><code>plot.asymmetry &lt;- ggplot(data=df.overview.month, 
                           aes(x=Date.Time, y=Asymmetry)) +    
        geom_area(data=subset(df.overview.month, Asymmetry &lt; 0),     
                           aes(x=Date.Time, y=Asymmetry)) 
  </code></pre>
  
  <p>However - as could be expected - this does not shade the area between geom_line and the x-axis, but only between negative values of the asymmetry data, which is something else entirely, as shown in the resulting graph:</p>
  
  <p><img src="https://i.stack.imgur.com/4nUMI.png" alt="enter image description here"></p>
  
  <p>Is there any way to overcome this problem?</p>
  
  <p>/Edit: some example data:</p>
  
  <pre><code>time.initial &lt;- as.POSIXct("2010-12-31 23:00:00", tz="GMT")
  Date.Time&lt;-vector()
  for(i in 1:24) {
  Date.Time[i] &lt;- time.initial + i*3600
  }
  
  Demand&lt;-vector()
  for(i in 0:23) {
  Demand[i+1] &lt;- 155 + 20*sin((pi/12)*i - (pi/2)) + 10*sin((pi/4380)*i + (pi/2))
  }
  
  Supply&lt;-vector()
  for(i in 0:23) {
  Supply[i+1] &lt;- 165 + 5*sin((pi/4380)*i - (pi/2)) + rnorm(1, mean=0, sd=0.20*165)
  }
  
  df.overview.month &lt;- data.frame(Date.Time, Demand, Supply, Asymmetry=Supply-Demand)
  </code></pre>