title: how do i add vectors to a (Vegan package) ordihull plot?
body: <p>how do i add vectors (illustrating annual change) to a ordihull plot?
  I have created a ordihull plot illustrating the years 2011, 2013 and 2015 each with it's own colour. now i would like to see if all locations change in the same direction or if they move randomly. statistically i found a significant change on axis 3, therefore i want to apply vectors amplifying this change. The change is not apparent from the plot as it is now.<a href="http://i.stack.imgur.com/UrE35.jpg" rel="nofollow">Ordihull plot of 3 years</a>  </p>
  
  <p>My script:</p>
  
  <pre><code>library(vegan)
  matrix_s = read.table('matrix_s.txt', sep = "\t", header = TRUE)
  matrixfit4 &lt;- metaMDS(matrix_s, k=4, trymax=50, distance="bray")
  ordiplot(matrixfit4,display = "site",type="p",choices = c(2,3))
  ordihull(matrixfit4, K4Full$Year, display = "sites", show.groups = "2011",col= c("purple"),choices = c(2,3), draw = "polygon")
  ordihull(matrixfit4, K4Full$Year, display = "sites", show.groups = "2013",col= c("red"), choices = c(2,3),draw = "polygon")
  ordihull(matrixfit4, K4Full$Year, display = "sites", show.groups = "2015",col= c("orange"),choices = c(2,3), draw = "polygon")
  legend(-1.195, 1.555, unique(K4Full$Year), text.col =c("purple","red","orange"),cex=1)
  </code></pre>