title: ggplot - adding vertical lines
body: <p>Below is the data.table I am working with. I want to plot verticle lines whenever longSignal column is 1. </p>
  
  <pre><code>data.frame(
          index = c("2011-09-09 17:00:00", 
                      "2011-09-12 17:00:00",
                      "2011-09-13 17:00:00", "2011-09-14 17:00:00",
                      "2011-09-15 17:00:00", "2011-09-16 17:00:00", "2011-09-19 17:00:00",
                      "2011-09-20 17:00:00", "2011-09-21 17:00:00",
                      "2011-09-22 17:00:00", "2011-09-23 17:00:00", "2011-09-26 17:00:00",
                      "2011-09-27 17:00:00", "2011-09-28 17:00:00", "2011-09-29 17:00:00",
                      "2011-09-30 17:00:00", "2011-10-03 17:00:00",
                      "2011-10-04 17:00:00", "2011-10-05 17:00:00", "2011-10-06 17:00:00",
                      "2011-10-07 17:00:00"),
     EURUSD.Close = c(1.36534, 1.367895, 1.36783, 1.37546, 1.38764, 1.38005,
                      1.36849, 1.37009, 1.35722, 1.346385, 1.35002, 1.353255,
                      1.35825, 1.35425, 1.359705, 1.33876, 1.31759, 1.33489, 1.33482,
                      1.34374, 1.33771),
       longSignal = c(0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0,
                      0)
  )
  </code></pre>
  
  <p>Here is the code that I was trying to work with in ggplot</p>
  
  <pre><code>ggplot(RSI_data, aes(index, EURUSD.Close)) +
    geom_line() + 
    geom_vline(aes(xintercept = as.numeric(RSI_data$index[which(RSI_data$longSignal == 1)]), 
               size = 2, colour = "red"))
  </code></pre>
  
  <p>I have been getting error. Could anyone tell me how can I do this right? I 
  Thanks in advance!</p>