title: add a simple legend on ggplot with linetype
body: <p>How do I add a legend to this ggplot? 
  I've searched everywhere but couldn't find a simple way that matches my current code.
  I've got three variable, they all have the same colour with different linetypes. </p>
  
  <pre><code>ggplot()+geom_line(data=datapop, aes(Year, OECD),size = 0.7, color="#69b3a2") +
      geom_line(data=datapop, aes(Year, World),size = 0.7, color="#69b3a2", linetype="dashed")+
      geom_line(data=datapop, aes(Year, Switzerland),size = 0.7, color="#69b3a2", linetype="twodash")+
      xlab("Years")+ ylab("Aging") +theme_minimal()+ labs(color="Legend text")+
      scale_x_date(date_breaks = "2 years",date_labels = "%Y")+
      theme(axis.text.x=element_text(angle=60, hjust=1))
  </code></pre>