title: ggplot2 boxplot facet wrap
body: <p>I am trying to use ggplot2 in R to construct a boxplot with different parameter sets in different panels or facets.  In my input datatable, I have a column "process" which contains the parameter groupings.  When I plot using facet_wrap, though, all of the parameters are displayed for each facet, even though most have no data, even when I use drop=T.</p>
  
  <p>Below is my code, any suggestions will be very helpful!</p>
  
  <pre><code>ggplot(stRep, aes(x=Parameter, y=value, fill=Disease), facets=process) + 
    geom_boxplot(outlier.shape=NA) + 
    ylab("Posterior distribution") + 
    xlab("Parameter") + theme_bw() + 
    scale_fill_grey() + coord_flip() +  
    ylim(-6, 10) + 
    facet_wrap( ~ process, drop=T,  ncol=1)
  </code></pre>
  
  <p>Attached is a subset of data:</p>
  
  <pre><code>&gt; test[c(1:5, 39995:40005),]
              value           Parameter Disease              process
  5001  -4.52611948 initial probability    tree   General parameters
  5002   6.73178928      pers.intercept    tree          Persistence
  5003   6.00318901      pers.intercept    tree          Persistence
  5004  -4.05923658   pers. nei. effect    tree          Persistence
  5005   0.05733596   pers. nei. effect    tree          Persistence
  39995 -0.10238927    col. tick effect    corn Initial colonization
  39996 -0.12752092    col. tick effect    corn Initial colonization
  39997 -0.17067746    col. tick effect    corn Initial colonization
  39998 -0.06580708    col. tick effect    corn Initial colonization
  39999 -0.13382417    col. tick effect    corn Initial colonization
  40000 -0.12990795    col. tick effect    corn Initial colonization
  40001  0.22196724    col. Lyme effect    corn Initial colonization
  40002  0.24598469    col. Lyme effect    corn Initial colonization
  40003  0.26436187    col. Lyme effect    corn Initial colonization
  40004  0.23429840    col. Lyme effect    corn Initial colonization
  40005  0.22931861    col. Lyme effect    corn Initial colonization
  </code></pre>