How can I change the legend linetype in ggplot2?

Question

I have this simple plot where I cannot change the linetype in the legend according to the linetype in the plot using scale_linetype_manual.

The code to produce the plot is :

library(ggplot2)

x_1 <- rep(0:6, each = 2)
pdf_1 <- c(0,0.05,0.05,0.1,0.1,0.15,0.15,0.3,0.3,0.25,0.25,0.15,0.15,0)

x_2 <- rep(3:9, each = 2)
pdf_2 <- c(0,0.05,0.05,0.1,0.1,0.15,0.15,0.3,0.3,0.25,0.25,0.15,0.15,0)

data_1 <- data.frame(x_1, pdf_1,x_2,pdf_2)

ggplot()+
  geom_line(data=data_1,aes(x=x_1, y=pdf_1, color="Forecaster_1"),linetype='solid',size=1)+
  geom_line(data=data_1,aes(x=x_2, y=pdf_2, color="Forecaster_2"),linetype='dashed',size=1)+
  labs(x = "x") +
  labs(y = "PDF") +
  scale_colour_manual(values = c('Forecaster_1' = 'cornflowerblue', 'Forecaster_2' = 'coral2')) +
  scale_linetype_manual(values = c('Forecaster_1' = 'solid', 'Forecaster_2' = 'dashed')) 

I would appreciate someone help me with this.

Answer 1

It would be better if you mapped the line type as an aethetic and then make sure you use the same name for both guides. For example

ggplot()+
  geom_line(data=data_1,aes(x=x_1, y=pdf_1, color="Forecaster_1", linetype='Forecaster_1'),size=1)+
  geom_line(data=data_1,aes(x=x_2, y=pdf_2, color="Forecaster_2", linetype='Forecaster_2'),size=1)+
  labs(x = "x") +
  labs(y = "PDF") +
  scale_colour_manual("Line", values = c('Forecaster_1' = 'cornflowerblue', 'Forecaster_2' = 'coral2')) +
  scale_linetype_manual("Line", values = c('Forecaster_1' = 'solid', 'Forecaster_2' = 'dashed'))

Answer 2

You could override the default legend like so:

ggplot() +
  geom_line(
    data = data_1,
    aes(
      x     = x_1,
      y     = pdf_1,
      color = "Forecaster_1"
    ),
    linetype = 'solid', 
    size = 1
  ) +
  geom_line(
    data = data_1,
    aes(
      x = x_2,
      y = pdf_2,
      color = "Forecaster_2"
    ),
    linetype = 'dashed',
    size = 1
  ) +
  labs(x = "x") +
  labs(y = "PDF") +
  scale_colour_manual(
    values = c('Forecaster_1' = 'cornflowerblue', 'Forecaster_2' = 'coral2'),
    guide = guide_legend(
      override.aes = list(
        linetype = c("solid", "dotted")
      )
    )
  )

linetype can be replaced with any of the values listed here .