title: Error when I try and use ggbiplot from the library github
body: <p>I am trying to use the plotting function ggbiplot using this code:</p>
  
  <pre><code>  install.packages("devtools")
    library(devtools)
    install_github("vqv/ggbiplot")
  
    g &lt;- ggbiplot(iris.pca, obs.scale = 1, var.scale = 1, 
              groups = iris[,5], ellipse = TRUE, 
              circle = TRUE)
   g &lt;- g + scale_color_discrete(name = '')
   g &lt;- g + theme(legend.direction = 'horizontal', 
               legend.position = 'top')
    print(g)
  </code></pre>
  
  <p>I get the following error:</p>
  
  <pre><code>Installation failed: there is no package called ‘R6’
  Error in library(ggbiplot) : there is no package called ‘ggbiplot’
  </code></pre>
  
  <p>I am trying to perform some PCA analysis following this demo <a href="https://www.r-bloggers.com/computing-and-visualizing-pca-in-r/" rel="nofollow noreferrer">https://www.r-bloggers.com/computing-and-visualizing-pca-in-r/</a></p>