title: Arguments for `vpList` from `grid` package
body: <p>This is probably silly but how do I dynamically create a <code>vpTree</code> like</p>
  
  
  
  <pre class="lang-r prettyprint-override"><code>library(grid)
  grid.newpage()
  vpTree( viewport(layout=grid.layout(2,2), name = "body"), vpList(viewport(name= "a"), viewport(name ="b"), viewport(name = "c"), viewport(name = "d")))
  #&gt; viewport[body]-&gt;(viewport[a], viewport[b], viewport[c], viewport[d])
  </code></pre>
  
  <p>I cannot pass a list of viewports:</p>
  
  
  
  <pre class="lang-r prettyprint-override"><code>library(grid)
  grid.newpage()
  n &lt;- c(viewport(name= "a"), viewport(name ="b"), viewport(name = "c"), viewport(name = "d"))
  vpTree( viewport(layout=grid.layout(2,2), name = "body"), vpList(n))
  #&gt; Error in vpListFromList(vps): only viewports allowed in 'vpList'
  </code></pre>
  
  <p>Any ideas?</p>