title: Conditional Graph in grid.arrange
body: <p>Is there a way to use a structure like </p>
  
  <pre><code>grid.arrange(
      ifelse(somecondition,graph1,graph2),
      ifelse(somecondition2,graph3,graph4),
      ncol=2
  )
  </code></pre>
  
  <p>where <code>graphX</code> is either a plot (created with <code>ggplot2</code>)  or a grob defined previously. It looks like <code>ifelse</code> evaluates the <code>grob</code> object to something else (a dataframe ?) before printing so <code>grid.arrange</code> doesn't get the right input to work properly.</p>
  
  <p>I also tried to store all the graph objects in a collection and use that within <code>grid.arrange</code> but  coudn't get a proper data structure to work nicely.</p>