title: How do I ad rings to my cmdscale plot?
body: <p>I have created a <a href="https://stat.ethz.ch/R-manual/R-devel/library/stats/html/cmdscale.html" rel="nofollow noreferrer">cmdscale plot</a>, using a modified script I got online, in R that plots my pigment data from phytoplankton samplings. </p>
  
  <p>(<a href="https://drive.google.com/open?id=0B_nGJOKiTVD6cVFNY2dVV01seEE" rel="nofollow noreferrer">https://drive.google.com/open?id=0B_nGJOKiTVD6cVFNY2dVV01seEE</a>)</p>
  
  <pre><code>   d &lt;- dist(fraktioneradRatio_15m2)
     fit &lt;- cmdscale(d, eig = TRUE, k = 4)
     x &lt;- fit$points[, 1]
     y &lt;- fit$points[, 2]
     plot(x, y, main="Fractions 15m")
     city.names &lt;- c("1", "2", "3",
                  "4", "5", "6",
                  "7", "9", "10",
                  "11", "12")
     text(x, y, pos = 4, labels = city.names)
  </code></pre>
  
  <p>The plot works fine and looks good. I now want to add rings with set distances to them so the distances between clusters can be easily identified. Is there any way I can do that? </p>