title: Is there a way to add a scale bar (for linear distances) to ggmap?
body: <p>Not that it's critical to my question, but here is my plot example, on top of which I'd like to add a scale bar.</p>
  
  <pre><code>ggmap(get_map(location = "Kinston, NC", zoom = 12, maptype = 'hybrid')) +
  geom_point(x = -77.61198, y = 35.227792, colour = "red", size = 5) +
  geom_point(x = -77.57306, y = 35.30288, colour = "blue", size = 3) +
  geom_point(x = -77.543, y = 35.196, colour = "blue", size = 3) +
  geom_text(x = -77.575, y = 35.297, label = "CRONOS Data") +
  geom_text(x = -77.54, y = 35.19, label = "NOAA") +
  geom_text(x = -77.61, y = 35.22, label = "PP Site")
  </code></pre>
  
  <p><img src="https://i.imgur.com/Z2NoAUu.jpg" alt="NC map"></p>