title: How to highlight area between two lines? ggplot
body: <p>I have data frame containing variable and it's conf. interval</p>
  
  <pre><code>time x     x.upper   x.lower
     1 1.00     0.91      1.11
     2 1.03     0.92      1.13
     3 1.03     0.95      1.17
     2 1.06     0.90      1.13
  </code></pre>
  
  <p>I <code>ggplot</code> it:</p>
  
  <pre><code>library(ggplot2)
  ggplot(data = df,aes(time,x))+
      geom_line(aes(y = x.upper), colour = 'red') +
      geom_line(aes(y = x.lower), colour = 'blue')+
      geom_line()
  </code></pre>
  
  <p>I want to highlight area between red and blue lines, smth similar to <code>geom_smooth()</code> function. How can I do it?</p>