title: ggplot update and not add a layer
body: <p>I know there are ways to update settings of a given ggplot graph.</p>
  
  <p>I want to save two files, one with the standard setting as png and another one as pdf but with a different label size.</p>
  
  <pre><code>df &lt;- data.frame(a=c('a;b;c','d;e;f'), b=c('A;B;C','D;E;F'),
                   x=c(1,2), y=c(2,3))
  
  g &lt;- ggplot(df, aes(x,y)) + geom_point() + geom_text(aes(label=a))
  
  ggsave('test1.png',g)
  ggsave('test2.pdf',g + geom_text(aes(label=a), size=10))
  </code></pre>
  
  <p>Is there a way to remove or update the old geom_text layer and not just add a layer to the graph?</p>