title: geom_density with alpha vector
body: <p>I'd like to plot several distributions with <code>geom_density</code>, and have all but one of them faded out.  How can I do this, as the following doesn't change the <code>alpha</code> as expected.</p>
  
  <pre><code>require("ggplot2")
  
  data = rbind(
    data.frame(generation=1, samples=rnorm(90)),
    data.frame(generation=2, samples=rnorm(100)),
    data.frame(generation=3, samples=rnorm(110)),
    data.frame(generation=4, samples=rnorm(120)),
    data.frame(generation=5, samples=rnorm(130)),
    data.frame(generation=6, samples=rnorm(140))
  )
  
  maxGen = max(data$generation)
  
  ggplot(data) + 
    geom_density(
      aes(
        colour= factor(generation), 
        x=samples,
        alpha=ifelse(data$generation == maxGen, 0.3, 1.0)
      )
    )
  </code></pre>
  
  <p>In case it makes a difference, the real thing adds a facet wrap across another variable.</p>