title: Change opacity on part of a geom_bar object
body: <p>I am trying to change the opacity of a ggplot bar plot. Consider the following example:</p>
  
  <pre><code>df &lt;- data.frame(period = rep(c("current", "past"), 3), 
                   type = c("so_far", "so_far", "source_x", "source_x", "source_y", "source_y"), 
                   income = c(12, 10, 7, 9, 4, 7))
  ggplot(df, aes(x = period, y = income, fill = type)) + geom_bar(stat = "identity")
  </code></pre>
  
  <p>This gives the following plot:
  <a href="https://i.stack.imgur.com/Hv3bU.jpg" rel="nofollow"><img src="https://i.stack.imgur.com/Hv3bU.jpg" alt="enter image description here"></a></p>
  
  <p>In reality though for the current period source_x and source_y are estimates, while in the past they are true values. So I want to alter the opacity of just the blue and green portions on the left side bar. 
  Can this be done? How?</p>