title: Add legend to geom_density R
body: <p>I'm working with the Prosper Loan dataset and I'm trying to show two variable in the same plot using geom_density.
  The problem, when I try to include the lengend to show the variable name from the pink area and the variable name from the dark area, it doesn't work.</p>
  
  <pre><code>library(ggplot2)
  EstimatedLoss &lt;-  c(0.5, 0.2,0.3,0.4,0.8,0.5, 0.2,0.3,0.4,0.8)
  EstimatedEffectiveYield &lt;- c(0.10, 0.15,0.18,0.20,0.8,0.15, 0.13,0.22,0.22,0.25)
  prosper_loan &lt;- data.frame(EstimatedLoss,EstimatedEffectiveYield)
  ggplot(data = prosper_loan)
  geom_density(aes(EstimatedLoss * 100), color = '#e1b582', fill = '#e1b582', alpha = 0.5, show.legend = TRUE ) +
  geom_density(aes(EstimatedEffectiveYield * 100), color = '#a2b285',fill = '#a2b285', alpha = 0.7, linetype = 3, size = 1, show.legend = TRUE) +
  scale_y_continuous(name = "Density")+
  scale_x_continuous(name = "Estimate loss and effective yield in percentage") +
  ggtitle('Density from the Estimated loss and effective yield in percentage') 
  </code></pre>
  
  <p>Am I doing anything wrong?</p>