title: changing title in multiplot ggplot2 using grid.arrange
body: <p>I have managed to make a 2x2 plot using grid.arrange:</p>
  
  <pre><code>library(gridExtra)
  grid.arrange(p1,p3,p2,p4, ncol=2, nrow=2, top = "Daily QC: Blue")
  </code></pre>
  
  <p>The main title of this multiplot is very small. Is there a way to change the title text size and font.</p>