title: Combined line & bar geoms: How to generate proper legend?
body: <p>The legend for <code>d2</code> looks fine; for <code>d1</code>, I would like to show just the hoizontal line against a white/transparent backgounnd.</p>
  
  <pre><code>df = data.frame(
    Date = c("2012-11-30", "2012-12-03", "2012-12-04"),
    d1 = c(9, 5, 11),
    d2 = c(4, 6, 3)
  )
  ggplot(df, aes(Date)) + 
    geom_bar(aes(y = d2, color = "d2"), stat="identity", fill = "red") +
    geom_line(aes(y = d1, group = 1, color = "d1"))  +
    scale_colour_manual("", values=c("d1" = "blue", "d2" = "red")) 
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/tI5V5.jpg" alt="enter image description here"></p>