title: R mathematical annotations
body: <p>I have a problem inserting a vertical line in a mathematical annotation. I have search through the annotation symbols without luck.</p>
  
  <p>I would like to put a mathematical annotation containing conditional probability sign (|) in the x-label of a plot. What I would like is the expression p(x_j | o_i) where "j" and "i" are subscripts.</p>
  
  <p>I have the following construction</p>
  
  <pre><code>plot(1:10, xlab = expression( paste("Likelihood, p" * (y[i] %up% o[j])) ))
  </code></pre>
  
  <p>and I wold like to substitute the arrow with a vertical line.
  Any help or guidance is greatly appreciated.</p>