title: R lattice: Change panel title layout
body: <p>I have a lattice graph with two conditions: one condition (x1) has 4 levels and one has 2 levels (x2). The graph has a stripes for each variable x1 and x2 (edit1: x3 is a third variable with 2 levels):</p>
  
  <pre><code>df &lt;- data.frame(y = runif(100,0,10)
                   , x1 = rep(c("A","B","C","D"),25)
                   , x2 = as.numeric(c(runif(100)&lt;0.5))
                   , x3 = c(runif(100) &lt; 0.5)
                   )
  df$x3 &lt;- with(df, ifelse( x3 == TRUE, "R","S"))
  histogram( ~y | x1 + as.factor(x2), data=df)
  </code></pre>
  
  <p>The upper panel contains the first level of x2, the lower panel the second level. The stripe for x1 is divided according the number of levels of x1 (vertical lines). This makes sense. But the stripe of x2 is also divided according the number of levels of x1 showing 4 times the level 0 respectively 1. I would like to see only one label for the stripe x1. This is what the code delivers:</p>
  
  <pre><code>0000
  ABCD
  </code></pre>
  
  <p>I would like to see this:</p>
  
  <pre><code>  0
  ABCD
  </code></pre>
  
  <p>Thanks for any help.</p>