title: Programmatically make colour slightly darker version of fill
body: <p>I'd like to highlight the bounday line of a density, using ggplot2, with the same colours as I use for fill, only slightly darker. Here is a small example:   </p>
  
  <pre><code>ggplot(chickwts, aes(x = weight)) +
    geom_density(bins = 25, aes(fill = feed, colour = feed)) +
    facet_wrap(~feed) +
    ylab(NULL) +
    theme_minimal() +
    theme(text = element_text(family = "Georgia", size = 12, face = "bold"),
          legend.position = "none")  
  </code></pre>
  
  <p>Which gives me the following plot:  </p>
  
  <p><a href="https://i.stack.imgur.com/LnH00.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/LnH00.png" alt=""></a></p>
  
  <p>What I would ideally like is to show the boundary line of the density in these same colours, but slightly darker. I could do this manually with <code>scale_colour_ manual()</code>, but I <em>really</em> don't want to do that. Is there a way to do this with ggplot2, perhaps with <code>alpha()</code>, or something like that?</p>