How to add a legend on a multiple line graph in R?

Question

I am trying to plot two different datasets on the same plot. I am using this code to add the lines and to actually plot everything

ggplot()+
geom_point(data=Acc, aes(x=Year, y=Accumulo), color="lightskyblue")+
geom_line(data=Acc, aes(x=Year, y=RM3), color="gold1")+
geom_line(data=Acc, aes(x=Year, y=RM5), color="springgreen3")+
geom_line(data=Acc, aes(x=Year, y=RM50), color="blue")+
geom_line(data=Vulcani, aes(x=Year, y=Accumulo.V), color="red")+
theme_bw()+
scale_x_continuous(expand=expand_scale(0)) + scale_y_continuous(limits=c(50,350),expand=expand_scale(0))

but I can't find any way to add a legend and add custom labels to the different series. I find a way to add legends on a single dataset, but I can't find a way to add to this one a legend on the side

Answer 1

You are better off creating a single dataset tailored to your plot needs before, which would be in the long format, so that you can give a single geom_line() instruction, and add colors to the lines with aes(color = ...) within the call to geom_line() . Here's an example with the midwest dataset (consider them as distinct datasets for the sake of example)

library(ggplot2)
library(dplyr)
library(tidyr)

long_midwest <- midwest %>% 
    select(popwhite, popasian, PID, poptotal) %>% 
    gather(key = "variable", value = "value", -PID, -poptotal) # convert to long format

long_midwest2 <- midwest %>% 
    select(poptotal, perchsd, PID) %>% 
    gather(key = "variable", value = "value", -PID, -poptotal)

plot_data <- bind_rows(long_midwest, long_midwest2) %>% # bind datasets vertically
    mutate(line_type = ifelse(variable == 'perchsd', 'A', 'B')) # creates a line_type variable

ggplot(data = plot_data, aes(x=poptotal, y = value))+
    geom_line(aes(color = variable, linetype = line_type)) +
    scale_color_manual(
        values = c('lightskyblue', 'gold1', 'blue'),
        name = "My color legend"
    ) +
    scale_linetype_manual(
        values = c(3, 1), # play with the numbers to get the correct styling
        name = "My linetype legend"
    )

I added a line_type variable to show the most generic case where you want specific mapping between the column values and the line type. If it is the same than, say, variable , just use aes(color = variable, linetype = variable) . You can then decide which linetype you want ( see here for more details ).

For customising the labels, just change the content of variable within the dataset with the desired values.