title: Adding lines to graph created using plotrix library
body: <p>I have created a stacked histogram using the <code>multhist</code> function in the <code>plotrix</code> library, but I am unable to add a straight line to this histogram. Code that I would normally use doesn't seem to work in this setting.</p>
  
  <p>Here's an example. I am trying to add the mean and standard errors of the overall distribution as simple vertical lines on the histogram, but these do not work properly. What am I doing wrong?</p>
  
  <pre><code>library(plotrix)
  
  test1&lt;-rnorm(30,0)
  test2&lt;-rnorm(30,0)
  test3&lt;-rnorm(30,0)
  
  forstats&lt;-c(test1,test2,test3)
  mn&lt;-mean(forstats)
  se&lt;-std.error(forstats)
  
  together&lt;-list(test1,test2,test3)
  multhist(together, col=c(7,4,2), space=c(0,0), beside=FALSE,right=FALSE)
  
  abline(v=mn)
  abline(v=mn+se)
  abline(v=mn-se)
  </code></pre>