title: Plotting PCA biplot with autoplot: modify arrow thickness
body: <p>I am using the <code>autoplot</code> function from <code>ggfortify</code> as illustrated by the code below using <code>iris.pca</code>. This example only has three variables (hence 3 loadings) but my data set has a lot more variables so I wish I could reduce the size of the arrows to make the plot more readable. I think the default size is 1 pt and I'd like to get it down to something like 0.5. Although there are arguments to modify the loadings color and label size, I could not find a way to adjust the thickness of the arrows. Does anyone knows how to do this?</p>
  
  <pre><code>library(ggplot2)
  library(ggfortify)
  
  df &lt;- iris[c(1, 2, 3, 4)]
  iris.pca&lt;-(prcomp(df))
  
  autoplot(iris.pca, data=iris, colour="Species", loadings=TRUE, loadings.colour = "black")+
    scale_colour_manual(values=c("forestgreen","red","blue"))+
    scale_fill_manual(values=c("forestgreen","red","blue"))+
    scale_shape_manual(values=c(25,22,23))+
    theme_bw()
  </code></pre>