title: ggplot graph stored erase each others
body: <p>Here is some code which reproduces my issue:</p>
  
  <pre><code>x &lt;- as.factor(1:20)
  y &lt;- 1:20
  id &lt;- as.factor(c(rep(0,19),1))
  g1 &lt;- ggplot() + geom_bar(stat = "identity", aes(x = x, y = y, color = id, fill = id), width = 0.5) + ggtitle("g1")
  g1 # First print
  
  y &lt;- 20:1
  g2 &lt;- ggplot() + geom_bar(stat = "identity", aes(x = x, y = y, color = id, fill = id), width = 0.5) + ggtitle("g2")
  g2
  g1 # Second print
  </code></pre>
  
  <p>As you can see when running the code above, the first time you print g1, you have a barplot starting at (factor 1, y = 1), ending at (factor 20, y = 20).
  After having created g2, if you print again g1, it looks the same than g2, except the title which isn't modified.</p>
  
  <p>I'm really puzzled, any help would be much appreciated !</p>