title: How to adjust relative transparency of ggplot2 points
body: <p>I want to make some points stand out on a ggplot2 chart by giving them less transparency while the rest fade to the background.  But no matter what two alpha values I give the sets of points, their relative transparency is the same.</p>
  
  <p>Here's <code>0.8</code> vs <code>0.7</code>:</p>
  
  <pre><code>x &lt;- mtcars
  x$opacity &lt;- ifelse(x$cyl == 6, 0.8, 0.7)
  
  ggplot(x, aes(x = wt, y = mpg, color = cyl, alpha = opacity)) +
    geom_point()
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/KCcVi.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/KCcVi.png" alt="ggplot2 scatterplot with 0.8 and 0.7 alpha for two sets of points"></a></p>
  
  <p>And here's <code>0.8</code> vs <code>0.1</code> -- looks the same:</p>
  
  <pre><code>x$opacity &lt;- ifelse(x$cyl == 6, 0.8, 0.1)
  
  ggplot(x, aes(x = wt, y = mpg, color = cyl, alpha = opacity)) +
    geom_point()
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/n3Wq4.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/n3Wq4.png" alt="ggplot2 scatterplot with 0.8 and 0.1 alpha for two sets of points"></a>
  How can I fine-tune that relative alpha so that the two sets are closer in transparency?  Right now the values of the two numbers don't seem to matter.  Specifically, in this case I want the darker points (with the higher alpha) to be more transparent.</p>