title: How can I hide the part of the legend using ggplot2?
body: <p>I am generating a plot using the code below and would like to hide the "mag" (size) legend but still keep the "depth" (color) legend.</p>
  
  <p><code>ggplot(quakes, aes(long, lat, size=mag, colour=depth)) + geom_point()</code></p>