How to add a legend to hline?

Question

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") 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.

Answer 1

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"))))