title: Add sub text to viewport
body: <p>I would like to add sub text (a <code>textGrob</code>) to a <code>grid</code> <code>Viewport</code> -- is this possible? </p>
  
  <p>I have figured out how to add sub text to a single chart: </p>
  
  <pre><code>require(ggplot2) 
  require(gridExtra)
  
  # make the data
  timeSeries &lt;- data.frame(date = seq(as.Date("2001-01-01"), as.Date("2012-01-01"), by = "mon"), 
                       value = 1:133 + rnorm(133, 0, 10)
                       )
  
  
  # make the ggplots
  gpLine &lt;- ggplot(timeSeries, aes(x = date, y = value)) +
                  geom_line()
  
  gpBar &lt;- ggplot(timeSeries, aes(x = date, y = value)) +
                  geom_bar(stat = 'identity')
  
  # ggplot + subtext
  grid.arrange(gpBar, sub = textGrob("why is this at the bottom?"))
  </code></pre>
  
  <p>... And i can stick the two charts together using a <code>grid</code> <code>Viewport</code></p>
  
  <pre><code># two plots, one view
  vplayout &lt;- function(x,y) viewport(layout.pos.row = x, layout.pos.col = y)
  
  pushViewport(viewport(layout = grid.layout(3,1)))
  print(gpLine, vp = vplayout(1:2, 1))
  print(gpBar, vp = vplayout(3, 1))
  </code></pre>
  
  <p>... but i cannot figure out how to add text to the bottom of the resulting viewport. </p>
  
  <p><code>Grid</code> is <em>so</em> complete, i'm sure there <strong>must be</strong> a way, but it's hidden to me. </p>