title: How to add a loop to another loop?
body: <p>I have the script below that is functioning. 
  But, I would like to have a loop for taking as y, different variables that are in columns; in this example : Petal.length and Petal.width.
  Since, there is already a loop in this script, I don't know how to add one.</p>
  
  <pre><code>data &lt;-iris
  uniq_species = unique(data$Species)
  plot_list = list()
  for (i in uniq_species) {
    temp_plot = ggplot(data= subset(data, Species == i),aes(x=Sepal.Length, y=Sepal.Width, colour=Species)) + 
      geom_boxplot()+ ggtitle(i)
    plot_list[[i]] =temp_plot}
  pdf("y=Sepal Width per species.pdf")
  for (i in uniq_species) {
    print(plot_list[[i]])}
  dev.off()
  </code></pre>