How to add a legend to hline?

I'd like to add a legend to hline plot.

The head of my subset looks like this

    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

My code is as follows:

require(ggplot2)
require(reshape2)
require(magrittr)
require(dplyr)
require(tidyr)

setwd("~/Documents/Results")
mydata <- read.csv("Metals sheet Rwosnb5.csv")

L <- subset(mydata,
            Site == "Bo1"| Site == "Bo2"| Site == "Bo3"| Site == "Bo4"| Site == "Bo5" |
            Site == "Bo6",
            select = c(Site, Date, Al))
L$Date <- as.Date(L$Date, "%d/%m/%Y")

I <- 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)

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.

You can use the linetype aesthetic to make a separate legend for the horizontal lines rather than adding them to the existing legend.

To do this we can move linetype inside aes while still mapping to a constant. I used your desired labels as the constant. The legend name and the line type used can be set in scale_linetype_manual. I remove show.legend = TRUE to keep the lines out of the other legend. The legend colors are fixed in override.aes.

I + geom_hline(aes(yintercept= 10, linetype = "NRW limit"), 
               colour= 'red') +
    geom_hline(aes(yintercept= 75.5, linetype = "Geochemical atlas limit"),
               colour= 'blue') +
    scale_linetype_manual(name = "limit", 
                          values = c(2, 2),
                          guide = guide_legend(override.aes = list(color = c("blue", "red"))))