title: how can I change the legend for ggbiplot?
body: <p>Actually I am trying to plot PCA by this package but when I plot the loading, I cannot change the legend as I wish (e.g. if I want to set the legend to (+)M it shows something else. 
  what I do is as follows: </p>
  
  <pre><code>library(ggbiplot)
  require(ggplot2)
  
  data(iris)
  data &lt;- data.frame(iris[,1:4])    
  ir.pca &lt;- prcomp(data, center = TRUE, scale. = TRUE)
  theta &lt;- seq(0,2*pi,length.out = 100)
  circle &lt;- data.frame(x = cos(theta), y = sin(theta))
  p &lt;- ggplot(circle,aes(x,y)) + geom_path()
  
  loadings &lt;- data.frame(ir.pca$rotation, 
                         .names = row.names(ir.pca$rotation))
  p + geom_text(data=loadings, 
                mapping=aes(x = PC1, y = PC2, label = .names, colour = .names)) +
    coord_fixed(ratio=1) +
    labs(x = "PC1", y = "PC2")
  </code></pre>
  
  <p>Now the problem is if you change the name of the variables to for example (+)C , (-)C, (*)C and (%)C 
  then plot, it shows something else in legend instead </p>
  
  <p>on the other hand,  the title of the legend is .name , how to also set this to something else?</p>