Adding a legend to a double plot using ggplot

Question

I'm trying to add a legend to my plot using ggplot in R. Everything OK so far. My case is special because I'm trying to deal with three variables, but not in order to draw a 3D plot but draw a 2D plot facing v1 vs. v2 and v1 vs. v3.

I get my plot in a correct way but I dont get the legend.

This is my code:

colfuncWarmest <- colorRampPalette(c("orange","red"))
colfuncColdest <- colorRampPalette(c("green","blue"))

plot <- ggplot(data=temperatures_Celsius, aes(x=temperatures_Celsius$Year))
params <- labs(title=paste("Year vs. (Warmest minimum temperature\n",
                      "and Coldest minimum temperature)"),
               x="Year",
               y="Coldest min temp / Warmest min temp")
theme <- theme(plot.title = element_text(hjust = 0.5)) #Centering title
wmtl<-geom_line(data=temperatures_Celsius,
                aes(y=temperatures_Celsius$Warmest.Minimum.Temperature..C.,
                    color="red"
                ),
                colour=colfuncWarmest(length(temperatures_Celsius$Year))
              )
wmtt<-stat_smooth(data=temperatures_Celsius,
                  aes(y=temperatures_Celsius$Warmest.Minimum.Temperature..C.),
                  color="green",
                  method = "loess")

cmtl<- geom_line(data=temperatures_Celsius,
                 aes(y=temperatures_Celsius$Coldest.Minimum.Temperature..C.,
                     color="blue"
                  ),
                 colour=colfuncColdest(length(temperatures_Celsius$Year))
                )
cmtt<-stat_smooth(data=temperatures_Celsius,
                  aes(y=temperatures_Celsius$Coldest.Minimum.Temperature..C.),
                  color="orange",
                  method = "loess")


plot + theme + params + wmtl +  wmtt  +  cmtl +  cmtt

(Not all code was added because I did a lot of changes. It is only to get an idea) I get this:

If I add

+ scale_color_manual(values=c("red","blue"))

(for example) in order to add the legend, I get no error, but nothing different happens. I get the same plot.

What I want is only two lines. A red one that says "Warmest minimum" and another blue line that says "Coldest minimum". What could I do to get my legend in this way?

Thanks in advance.

Answer 1

Generally I would say that the correct way to apply a legend to a ggplot is to map a variable to an aesthetic (such as fill, color, size, alpha). Usually this consists of transforming the data to long format (key ~ value pair) and mapping the key variable to color or other aestetic.

In the current case this is not desirable since there is next to no chance the color gradient ( colorRampPalette ) on the line could be achieved. So I suggest a hacky way where a dummy layer (layer which will not be seen on the plot) is used to create the legend.

Here is some data

temperatures_Celsius = data.frame(year = 1900:2000,
                        Warmest = rnorm(100, mean = 20, sd = 5),
                        Coldest = rnorm(100, mean = 10, sd = 5))

Your plot:

colfuncWarmest <- colorRampPalette(c("orange","red"))
colfuncColdest <- colorRampPalette(c("green","blue"))


plot <- ggplot(data=temperatures_Celsius, aes(x=year))
params <- labs(title=paste("Year vs. (Warmest minimum temperature\n",
                           "and Coldest minimum temperature)"),
               x="Year",
               y="Coldest min temp / Warmest min temp")
theme <- theme(plot.title = element_text(hjust = 0.5)) #Centering title

wmtl<-geom_line(data=temperatures_Celsius,
                aes(y=Warmest),
                colour=colfuncWarmest(length(temperatures_Celsius$year)))

wmtt<-stat_smooth(data=temperatures_Celsius,
                  aes(y=Warmest),
                  color="green",
                  method = "loess")

cmtl<- geom_line(data=temperatures_Celsius,
                 aes(y=Coldest),
                 colour=colfuncColdest(length(temperatures_Celsius$year)))

cmtt<-stat_smooth(data=temperatures_Celsius,
                  aes(y=Coldest),
                  color="orange",
                  method = "loess")

plot1 <- plot + theme + params + wmtl +  wmtt  +  cmtl +  cmtt

Now add a dummy layer:

plot1+
geom_line(data = data.frame(year = c(1900, 1900),
                   group = factor(c("Coldest", "Warmest"), levels = c("Warmest", "Coldest")),
                   value = c(10, 20)), aes(x=year, y = value, color = group), size = 2)+
  scale_color_manual(values=c("red","blue"))