title: Display percentage values on a pie chart
body: <p>I am trying to draw a pie chart with ggplot2. My code is shown below.</p>
  
  <pre><code>df &lt;- data.frame(
    variable = c("australia","hungary","germany","france","canada"),
    value = c(632,20,491,991,20)
  )
  ggplot(df, aes(x = "", y = value, fill = variable)) +
    geom_bar(width = 1, stat = "identity") +
    scale_fill_manual(values = c("red", "yellow","blue", "green", "cyan")) +
    coord_polar(theta = "y") +
    labs(title = "pie chart")
  </code></pre>
  
  <p>I would like to  display percentage values. How can I do that? </p>