title: Draw two groups of multiple vertical lines in ggplot
body: <p>I want to drew the two groups such as <code>a1, a2</code> and <code>b1, b2</code> of vertical lines in ggplot using the following <code>r</code> functions.</p>
  
  <pre><code>myline = data.frame(vv = c(a1 = 25, a2 = 28, b1 = 52, b2 = 53))
  
  
  set.seed(100)
  d = data.frame(y = c(rnorm(100,5,1), rnorm(100, 2,4)), x = 1:200)
  ggplot(data = d) + geom_line(aes(x, y), color = "steelblue") + 
    geom_vline(data = myline, aes(xintercept=as.numeric(vv)), col= 'red', size = 0.8)
  </code></pre>
  
  <p>I am trying to separate the <code>a</code> and <code>b</code> groups in different colors. How can I do that? Thank you very much for your suggestions.</p>