title: Modifying the shape for a subset of points with ggplot2
body: <p>I'm attempting to plot a large scatter plot that varies along a large number of dimensions. </p>
  
  <p>Here's my starting plot:</p>
  
  <pre><code> p &lt;- ggplot(mtcars, aes(wt, mpg, shape=cyl, colour=gear, size=carb)) +
        geom_point()
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/g9voh.png" alt="Sample plot with multiple variables"></p>
  
  <p>Using the <code>mtcars</code> dataset, I'm simply including various shapes, colours, and sizes.  Now what i'd like to add to this is some distinguishing mark around all points with carb==8.  For example, add in a big black circle around these points.</p>
  
  <p>Doing </p>
  
  <pre><code>p + geom_point() + geom_point(colour="black", shape=1, size=5)
  </code></pre>
  
  <p>is close in that it gets the circles as I want, but all points get circles, I want to instead limit this to just some of the points.</p>
  
  <p>I would greatly appreciate any suggestions for this problem.</p>