title: Two lines of X axis labels in ggplot
body: <p>I want to make two lines of X axis labels in ggplot.</p>
  
  <p><img src="https://i.stack.imgur.com/ssKw7.png" alt="enter image description here"></p>
  
  <p>In this plot, I want to add one more line of label below each specified year. Something like </p>
  
  <p><code>1990 1995 2000 2005 2010
   cold warm warm cold warm</code></p>
  
  <p>This is my code for making this plot</p>
  
  <pre><code>ggplot(subset(dat, countryid %in% c("1")),  aes(date, 
  nonpartisan))+geom_line(aes(color=countryid), color="dodgerblue1", 
  size=1.4)+geom_line(aes(date, reshuffle), color="gray")+ theme_bw()
  </code></pre>
  
  <p>Is there any way to make one more line of label by creating a column specifically for the labels?</p>
  
  <p>Thanks!</p>