title: Adding vertical line in plot ggplot
body: <p>I am plotting a graph using the following piece of code:</p>
  
  <pre><code>library (ggplot2)
  
  png (filename = "graph.png")
  stats &lt;- read.table("processed-r.dat", header=T, sep=",")
  attach (stats)
  stats &lt;- stats[order(best), ]
  sp &lt;- stats$A / stats$B
  index &lt;- seq (1, sum (sp &gt;= 1.0))
  stats &lt;- data.frame (x=index, y=sp[sp&gt;=1.0])
  ggplot (data=stats, aes (x=x, y=y, group=1)) + geom_line()
  dev.off ()
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/XGS1b.png" alt="enter image description here"></p>
  
  <p>1 - How one can add a vertical line in the plot which intersects at a particular value of y (for example 2)?</p>
  
  <p>2 - How one can make the y-axis start at 0.5 instead of 1?</p>