title: Strange performance of multi qplots to a figure in R when parameter of qplot includes list
body: <p>Aim: save multi qplots into a figure. 
  The point is there is a list parameter to loop (see code. It's more clear!). The output is different as I expected. They are the same!</p>
  
  <pre><code># Make y be a list containing different values.
  y &lt;- list()
  for (j in 1:6) {
      y[[j]] &lt;- rnorm(10)
  }
  
  # plot multi qplots into one figure
  plots &lt;- list()  # new empty list
  for (i in 1:6) {
      p1 = qplot(1:10, y[[i]],  main = i)
      plots[[i]] &lt;- p1  # add each plot into plot list
  }
  do.call(grid.arrange, plots)
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/aylPi.png" alt="The figure"></p>
  
  <p>The code is revised from <a href="http://rstudio-pubs-static.s3.amazonaws.com/2852_379274d7c5734f979e106dcf019ec46c.html" rel="nofollow noreferrer">http://rstudio-pubs-static.s3.amazonaws.com/2852_379274d7c5734f979e106dcf019ec46c.html</a></p>
  
  <p>There is <code>y</code> partly: (to illustrate they are different!). The figure only used the last one in y. It's strange.</p>
  
  <pre><code>[[1]]
   [1]  2.01846525 -2.32504052 -1.07201485 -0.21105479  0.25706024  0.50934754
   [7]  0.39844954  0.18110421  1.03368602  0.01185971
  
  [[2]]
   [1]  0.01824317 -1.51801208  1.68385158 -0.30159404 -0.34894329  0.62840458
   [7] -0.45447576  1.18625774 -0.36671100 -0.05502285
  ...
  [[6]]
   [1]  0.1134854  0.1806742 -0.9491033  0.7279389 -0.2193326  0.1595183 -1.1751557
   [8] -0.4416456 -0.7074360 -0.3887882
  </code></pre>