title: Error: Discrete value supplied to continuous scale
body: <p>I'm using ggmap to work on a map of madagascar</p>
  <pre><code>myMap &lt;- get_map(location=k, source=&quot;stamen&quot;, maptype=&quot;toner&quot;, crop=FALSE, 
                   zoom=16)
  </code></pre>
  <p>and to plot points on that map from an x y lat/lon grid</p>
  <pre><code>ggmap(myMap) + geom_point(data=GPS, aes(x = 'Lon', y ='Lat'))
  ggplot(data=GPS, aes(x=Lon,y=Lat)) + geom_point()
  </code></pre>
  <p>from data that look like this</p>
  <pre><code> Tree.ID       Type       Species       Lat      Lon Foraged Plot
         7   deadwood      Dracaena -21.37413 47.86700       N    1
         8   deadwood       Bivinia -21.37408 47.86696       N    1
         9   deadwood Beilschmiedia -21.37396 47.86691       N    1
        10 live trunk        Ocotea -21.37410 47.86690       N    1
        12   deadwood   Tambourissa -21.37418 47.86696       N    1
        13 live trunk      Canarium -21.37422 47.86691       N    1
  </code></pre>
  <p>But I get this error:</p>
  <blockquote>
  <p>Error: Discrete value supplied to continuous scale</p>
  </blockquote>
  <p>What should I do?</p>