title: Position ggplot title at the top right of the plot
body: <p>I am using the excellent <code>theme_minimal()</code> found in ggplot0.9.3 which has a white background. I would like to place the title of my plots in a custom location at the top right corner of the plot. In the following example I know the <code>x</code> and <code>y</code> values, but I am wondering if there is a way to pass <code>xmax</code> and <code>ymax</code> values to ensure text placement in the top-right. Ideally, the text would be right justified. </p>
  
  <pre><code>#example plot
  p &lt;- qplot(mpg, wt, data = mtcars, colour = cyl)
  p +  annotate("text", x = 30, y = 5, label = "Custom Title")
  
  #what I would like
  p + annotate("text", y= ymax, x =xmax_or_RightJustified)
  </code></pre>