R - manually adding legend to ggplot : Consider the graph below that I created with the code further below.  I would like to add a legend that might say something like "median" and "90% confidence interval."  I've seen lots of examples of how one can create complex datasets and then use scale properties, but in this case, I'd really hope to just be able to specify the legend directly somehow if possible.

[![enter image description here][1]][1]

    library(ggplot2)
    middle = data.frame(t=c(0,1,2,3),value=c(0,2,4,6))
    ribbon = data.frame(t=c(0,1,2,3),min=c(0,0,0,0),max=c(0,4,8,12))
    g = ggplot()
    g = g + geom_line  (data=middle,aes(x=t,y=value),color='blue',size=2)
    g = g + geom_ribbon(data=ribbon,aes(x=t,ymin=min,ymax=max),alpha=.3,fill='lightblue')
    print(g)

  [1]: http://i.stack.imgur.com/2KAV6.png