title: R vegan RDA not all levels of constraint are displayed in triplot
body: <p>In my RDA triplot I would like to display 'sites', 'species' and their constraints which in my case are Field and Trt. The problem is that not all levels of the constraints are displayed in the plot. There are two levels of each factor.</p>
  
  <p>My RDA code is:</p>
  
  <pre><code>Dummy.rda &lt;- rda(species.rda ~ Field + Trt,RDA.env, scale=TRUE)
  
  summary(Dummy.rda, scaling=3)  #Here I see only one level of each reported in:Biplot scores for constraining variables. However all levels appear in: Centroids for factor constraints
  
  anova.cca(Dummy.rda, step=100, by='margin') # degrees of freedom are correct for both factors (df=1)
  
  plot(Dummy.rda, scaling = 3) #This displays all levels of Field and Trt but only one of each has an arrow
  
  plot(Dummy.rda, display = "species", xlim = xlims, ylim = ylims, 
         scaling = 3)
  text(Dummy.rda, scaling = 3, display = "bp")  # I want to customize the RDA plot, but this 'text' only displays 1 level of each of Field and Trt.
  </code></pre>