title: Create a partial dashed line in ggplot2
body: <p>I'm creating a plot in R and need to create a line where some of the values are projections. The projections are represented as a dashed line. Here's the code:</p>
  
  <pre><code>df = data.frame(date=c(rep(2008:2013, by=1)),
                  value=c(303,407,538,696,881,1094))
  
  
  ggplot(df, aes(date, value, width=0.64)) + 
          geom_bar(stat = "identity", fill="#336699", colour="black") +
          ylim(c(0,1400)) + opts(title="U.S. Smartphone Users") +
          opts(axis.text.y=theme_text(family="sans", face="bold")) +
          opts(axis.text.x=theme_text(family="sans", face="bold")) +
          opts(plot.title = theme_text(size=14, face="bold")) +
          xlab("Year") + ylab("Users (in millions)") +        
          opts(axis.title.x=theme_text(family="sans")) +
          opts(axis.title.y=theme_text(family="sans", angle=90)) +
          geom_segment(aes(x=2007.6, xend=2013, y=550, yend=1350), arrow=arrow(length=unit(0.4,"cm")))
  </code></pre>
  
  <p>So I've created a line which extends from 2008 to 2013. However, I want a solid line from 2008 to 2011, and a dashed line from 2011 to the end. Do i just do two seperate line segments, or is there a seperate command I can use to get the desired result.</p>