title: What do hjust and vjust do when making a plot using ggplot?
body: <p>Every time I make a plot using ggplot, I spend a little while trying different values for hjust and vjust in a line like </p>
  
  <pre><code>+ opts(axis.text.x = theme_text(hjust = 0.5))
  </code></pre>
  
  <p>to get the axis labels to line up where the axis labels almost touch the axis, and are flush against it (justified to the axis, so to speak). However, I don't really understand what's going on. Often, <code>hjust = 0.5</code> gives such dramatically different results from <code>hjust = 0.6</code>, for example, that I haven't been able to figure it out just by playing around with different values. </p>
  
  <p>Can anyone point me to a comprehensive explanation of how hjust and vjust options work?</p>