title: How to add a legend to hline?
body: <p>I'd like to add a legend to hline plot. </p>
  
  <p>The head of my subset looks like this </p>
  
  <pre><code>Site       Date    Al
  1   Bo6 2014-10-07 152.1
  2   Bo1 2014-10-07 157.3
  3   Bo3 2014-10-07 207.1
  4   Bo4 2014-10-07 184.3
  5   Bo5 2014-10-07  23.2
  13  Bo6 2014-10-14  96.8
  </code></pre>
  
  <p>My code is as follows:</p>
  
  <pre><code>require(ggplot2)
  require(reshape2)
  require(magrittr)
  require(dplyr)
  require(tidyr)
  setwd("~/Documents/Results")
  mydata &lt;- read.csv("Metals sheet Rwosnb5.csv")
  mydata &lt;- read.csv("Metals sheet Rwosnb5.csv")
  L &lt;- subset(mydata, Site =="Bo1"| Site == "Bo2"| Site == "Bo3"| Site ==          "Bo4"| Site == "Bo5" | Site == "Bo6", select = c(Site,Date,Al))
  L$Date &lt;- as.Date(L$Date, "%d/%m/%Y")
  I &lt;- ggplot(data=L, aes(x=Date, y=Al, colour=Site)) +
    geom_point() + 
    labs(title = "Total Al in the Barlwyd and Bowydd in Pant-yr-afon    sites B4-B9
     2014-2015.", x = "Month 2014/2015",
     y = "Total concentration (mg/L)") +
    scale_y_continuous(limits = c(0, 500)) +
    scale_x_date(date_breaks = "1 month", date_labels = "%m")
  I + geom_hline(aes(yintercept= 10),  linetype = 2, colour= 'red',   show.legend =TRUE) +
    geom_hline(aes(yintercept= 75.5), linetype = 2, colour= 'blue', show.legend = TRUE)
  </code></pre>
  
  <p>For some reason the legend does not work -- the legend has the six sites with a line through them. I would ideally like a legend with title = limit
  and Label 1  (10) = NRW limit and  label 2 (75.5)= Geochemical atlas limit. </p>