title: Remove connecting lines in ggplot2 geom_polygon
body: <p>I've made the map below by subsetting a shapefile downloaded from www.gadm.org :</p>
  
  <pre><code>load(url('http://gadm.org/data/rda/GBR_adm0.RData'))
  library(ggplot2)
  ukMapFort &lt;- fortify(gadm)
  ukMapFortSub &lt;- subset(ukMapFort, lat &gt; 55.575 &amp; lat &lt; 55.739 &amp; long &gt; -1.929 &amp; long &lt; -1.7)
  ggplot() + geom_polygon(data=data.frame(ukMapFortSub), aes(long, lat, group=id), fill=NA, color="black")
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/zFftd.jpg" alt="enter image description here"></p>
  
  <p>How can I remove the two red lines? Note I've coloured the lines red using Photoshop - these lines are produced by R code, but are not coloured red by R code.</p>