Why geom_line legend if show.legend = FALSE and why different colours-CodePudding

After executing the code here below, I was wondering:

1- Why "A.line" and "B.line" variables appear in the geom_point() legend.

2- why there are four colors in the legend.

I guess both answers are related, but I can not tell what is going on.

I would like to have the legend just with "A.points" and "B.points".

I would also like the same colors in both lines and points (I guess this I can do manually).

Thanks in advance for your help.

Best, David

data.frame(x = rep(1:2,2),
           names.points = rep(c("A.point","B.point"), 2),
           y.point = c(2, 4, 7, 9),
           names.lines = rep(c("A.line","B.line"), each = 2),
           y.line = c(3, 3, 8, 8)) %>% 
  ggplot()  
  geom_point(aes(x = x, y = y.point, group = names.points, colour = names.points), size = 5)  
  geom_line(aes(x = x, y = y.line, group = names.lines, colour = names.lines), show.legend = FALSE)

CodePudding user response：

Legends are not related to geoms but to the scales and display the categories (or the range of the values) mapped on an aesthetic. Hence, you get four colors because you have four categories mapped on the color aesthetic. The geoms used are only displayed in the legend key via the so called key glyph which is a point for geom_point and a line for geom_line. And show.legend=FALSE only means to not display the key glyph for geom_line in the legend key, i.e. the legend keys shows only a point but no line.

To remove the categories related to the lines from your legend use e.g. the breaks argument of scale_color_discrete instead.

library(ggplot2)
library(dplyr)

data.frame(
  x = rep(1:2, 2),
  names.points = rep(c("A.point", "B.point"), 2),
  y.point = c(2, 4, 7, 9),
  names.lines = rep(c("A.line", "B.line"), each = 2),
  y.line = c(3, 3, 8, 8)
) %>%
  ggplot()  
  geom_point(aes(x = x, y = y.point, group = names.points, colour = names.points), size = 5)  
  geom_line(aes(x = x, y = y.line, group = names.lines, colour = names.lines), show.legend = FALSE)  
  scale_color_discrete(breaks = c("A.point", "B.point"))

UPDATE To fix your issue with the colors you could use a named color vector:

pal_col <- rep(c("darkblue","darkred"), 2)
names(pal_col) <- c("A.point", "B.point", "A.line", "B.line")

data.frame(
  x = rep(1:2, 2),
  names.points = rep(c("A.point", "B.point"), 2),
  y.point = c(2, 4, 7, 9),
  names.lines = rep(c("A.line", "B.line"), each = 2),
  y.line = c(3, 3, 8, 8)
) %>%
  ggplot()  
  geom_point(aes(x = x, y = y.point, group = names.points, colour = names.points), size = 5)  
  geom_line(aes(x = x, y = y.line, group = names.lines, colour = names.lines), show.legend = FALSE)  
  scale_color_manual(breaks = c("A.point", "B.point"), 
                     values = pal_col)