title: Different alpha values by facet with facet_wrap in ggplot
body: <p>I have two graphs from a single long data  frame that is faceted based on one of the columns. This produces one facet plot with way way more observations than the other. I want to set a low alpha value for the facet with far more observations but preserve the default alpha for the facet with fewer observations. </p>
  
  <p>Here is a reproduction of my code. </p>
  
  <pre><code>df &lt;- data.frame(spons=sample(c(1:100),10000, replace=T), variable='score', value=runif(10000, -1,1), time=rep(1:10, 1000), DataSub=sample(c('original', 'matched'),10000,replace=T,  prob=c(0.01, 0.99)))
  p &lt;- ggplot(df, aes(x=factor(time), y=factor(spons)))
  p+geom_point()+facet_grid(DataSub~., ncol=2)
  </code></pre>
  
  <p>What I want it to look like is the more dense of the plots to have alpha= 0.01 or something like that but leave the more sparse of the plots alone. </p>
  
  <p>Thanks. </p>