Facets with own legend each but consistent colour mapping

Question

I'd like to plot facets where each facet has its own legend, using code from this answer by joran. The colours of the points should be mapped to a factor, but not every facet contains every factor level, thus the colours/labels are assigned inconsistently. Can I fix that somehow?

dat <- structure(list(group1 = structure(c(1L, 1L, 2L, 2L), .Label = c("A", "B"), class = "factor"), group2 = structure(c(1L, 2L, 1L, 3L), .Label = c("a", "b", "c"), class = "factor"), x = 1:4, y = 1:4), .Names = c("group1", "group2", "x", "y"), row.names = c(NA, -4L), class = "data.frame")

  group1 group2 x y
1      A      a 1 1
2      A      b 2 2
3      B      a 3 3
4      B      c 4 4

Plot with only one legend:

library(ggplot2)
ggplot(dat) +
  geom_point(aes(x=x, y=y, colour=group2)) +
  facet_wrap(~group1) +
  scale_colour_manual(values=c("green", "red", "blue"), labels=c("green", "red", "blue"))

Plot with legend for each facet but "wrong" colours/labels:

library(gridExtra)
dat <- split(dat, f = dat$group1)

p1 <- ggplot(dat$A) +
  geom_point(aes(x=x, y=y, colour=group2)) +
  facet_wrap(~group1) +
  scale_colour_manual(values=c("green", "red", "blue"), labels=c("green", "red", "blue"))

p2 <- p1 %+% dat$B

grid.arrange(p1, p2, ncol=2)

score 4 · Accepted Answer · answered Apr 04 '16 at 14:42

Always use named vectors in scale_*_manual if you specify values in order to ensure correct mapping:

p1 <- ggplot(dat$A) +
  geom_point(aes(x=x, y=y, colour=group2)) +
  facet_wrap(~group1) +
  scale_colour_manual(values=c(a = "green", b = "red", c = "blue"), 
                      labels=c(a = "green", b = "red", c = "blue"),
                      drop = FALSE)

p2 <- p1 %+% dat$B

grid.arrange(p1, p2, ncol=2)

Facets with own legend each but consistent colour mapping

1 Answers1