title: ggplot2: How do you control point color with geom_jitter and geom_point?
body: <p>This is related to the question <a href="https://stackoverflow.com/questions/17560036/controlling-point-colors-with-geom-jitter-and-geom-boxplot-in-ggplot2-in-r">here</a>. However, I'm not trying to make a <strong>boxplot</strong>, but a <strong>scatterplot in ggplot2</strong>, but adding the argument <code>geom_jitter()</code> adds <strong>black dots</strong> which seem to be non-related to my dataset. </p>
  
  <p>Here's an <strong>example using the mpg data pack</strong>:</p>
  
  <p>This is a <strong>simple scatterplot, that looks a bit "too clean"</strong></p>
  
  <pre><code>gmpg&lt;-ggplot(data=mpg, aes(x=hwy, y=cty))
  gmpg+geom_point(aes(col=manufacturer))
  </code></pre>
  
  <p><strong>Which produces this:</strong>
  <img src="https://i.stack.imgur.com/ZWVH2.png" alt="Regular scatterplot, with no jitter"></p>
  
  <p>Now, if I add the argument jitter, this is what happens</p>
  
  <pre><code>gmpg+geom_point(aes(col=manufacturer))+geom_jitter()
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/tm3wP.png" alt="plot with geom_jitter"></p>
  
  <p>I've tried reducing the alpha etc., but the black dots remain. What on earth are these and how do I remove them? </p>