title: R lattice levelplot - combine many plots together with one color key on same scale
body: <p>I am trying to plot a 3x3 grid of levelplots which are all on the same color scale with one combined color key.</p>
  
  <p>I have found <code>latticeExtra</code> and its ability to override <code>c</code> and combine <code>trellis</code> objects together, with the <code>merge.legends</code> flag. However, when I do this I get multiple color keys.</p>
  
  <p>is a picture of what it looks like with only three of the plots.
  <img src="https://i.stack.imgur.com/0V6qr.png" alt="Here"> 
  And here is the code where I do this:</p>
  
  <pre><code>t1 &lt;- levelplot(counts[[1]], main="", col.regions=colorRampPalette(c("white","red"))(256))
  t2 &lt;- levelplot(counts[[2]], main="", col.regions=colorRampPalette(c("white","red"))(256))
  t3 &lt;-  levelplot(counts[[3]], main="", col.regions=colorRampPalette(c("white","red"))(256))
  
  plots &lt;- c(t1, t2, t3, merge.legends=T)
  print(plots)
  </code></pre>
  
  <p>I also would ideally like to not have to manually make each <code>trellis</code> object its own variable, but rather a member of a list:</p>
  
  <pre><code>plots &lt;- list()
  for (i in 1:length(counts)){
    if (i %% 3 == 0) {
      plots[[i]] &lt;- levelplot(counts[[i]], main="", col.regions=colorRampPalette(c("white","red"))(256))
    }
  }
  plots &lt;- c(unlist(plots), merge.legends=T)
  </code></pre>
  
  <p>But when I try to do this it seems that the <code>c</code> function is not overridden correctly. I have tried not using <code>unlist</code>, as well as making <code>plots</code> a vector, and they do not work.</p>