title: Manually adjust alpha after plot is created?
body: <p>Hi I would like to manually adjust the alpha of ggplot object.  There is function scale_alpha_manual but it does not seem to affect the color.  Here is an code example. </p>
  
  <pre><code>p &lt;- ggplot(mtcars, aes(mpg, wt)) +
      geom_point(aes(colour = factor(cyl)), size=12)
  p + scale_colour_manual(values = c("red", "blue", "green")) +
      scale_alpha_manual(values = c(1, .5, .1)) # does nothing
  </code></pre>
  
  <p>edit: the issue is that the ggplot has already been created and I want to manipulate the alpha after the fact.  I also tried to change the label, for example, <code>p$labels$alpha = "factor(cyl)"</code> but that does not work neither. </p>
  
  <p><a href="https://i.stack.imgur.com/do8sb.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/do8sb.png" alt="enter image description here"></a></p>