ggplot2: Assign color to 2 different geoms and get

Question

How can I make ggplot2 give a separate legend for different geoms that both use color to represent 2 different variables. I'd want 3 legends, one for z, a, and b, but a & b seem to be combined into a singe legend even though a & b represent different variables. I'd also like to be able to control the colors in each legend.

dat <- data.frame(
    y = rnorm(200),
    x = sample(c("A", "B"), 200, TRUE),
    z = sample(100:200, 200, TRUE), 
    a = sample(c("male", "female"), 200, TRUE),
    b = factor(sample(1:2, 200, TRUE))
)

ggplot(dat, aes(y = y, x = x)) +
    geom_point(aes(color = a, size = z)) + 
    geom_boxplot(fill = NA, size=.75, aes(color=b)) +
    scale_color_manual(values = c("#F8766D", "#00BFC4", "orange", "purple"))

Answer 1

If you use a filled plotting symbol, you can map one factor to fill and the other to colour, which then separates them into two scales and, therefore, legends.

ggplot(dat, aes(y = y, x = x)) +
  geom_point(aes(fill = a, size = z), pch = 21) + 
  geom_boxplot(fill = NA, size=.75, aes(color=b)) +
  scale_color_manual(values = c("orange", "purple")) +
  scale_fill_manual(values = c("#F8766D", "#00BFC4"))

Answer 2

Seems that the legend capture approach is the most generalizable in similar situations, though in this specific on @jennybryan's is simpler and probably what most people would want. I document the legend capture approach here as well. I first learned this approach from @Sandy Muspratt HERE.

dat <- data.frame(
    y = rnorm(200),
    x = sample(c("A", "B"), 200, TRUE),
    z = sample(100:200, 200, TRUE), 
    a = sample(c("male", "female"), 200, TRUE),
    b = factor(sample(1:2, 200, TRUE))
)

if (!require("pacman")) install.packages("pacman")
pacman::p_load(ggplot2, grid, gridExtra, gtable)

coldot <- ggplot(dat, aes(y = y, x = x)) +
    geom_point(aes(color = a, size = z)) + 
    #geom_boxplot(fill = NA, size=.75, aes(color=b)) +
    scale_color_manual(values = c("#F8766D", "#00BFC4"))

colbox <- ggplot(dat, aes(y = y, x = x)) +
    #geom_point(aes(color = a, size = z)) + 
    geom_boxplot(fill = NA, size=.75, aes(color=b)) +
    scale_color_manual(values = c("orange", "purple"))



leg1 <- gtable_filter(ggplot_gtable(ggplot_build(coldot)), "guide-box") 
leg1Grob <- grobTree(leg1)

leg2 <- gtable_filter(ggplot_gtable(ggplot_build(colbox)), "guide-box") 
leg2Grob <- grobTree(leg2)


noleg <- ggplot(dat, aes(y = y, x = x)) +
    geom_point(aes(color = a, size = z)) + 
    geom_boxplot(fill = NA, size=.75, aes(color=b), position=position_dodge(1)) +
    scale_color_manual(values = c("orange", "purple", "#F8766D", "#00BFC4")) +
    theme(
        plot.margin = unit(c(5.1, 4.1, 4.1, 2.1), "pt"),
        legend.position=c(1.3, 0.87)
    ) +
    guides(color = FALSE)

legs <- ggplot(data = data.frame(x=1, y=1)) +
    geom_blank(aes(x=x, y=y)) + 
    theme_minimal() + 
    ylab(NULL) + xlab(NULL) +
    theme(
        axis.text = element_blank(),
        axis.ticks = element_blank(),
        panel.grid.major = element_blank(),
        panel.grid.minor = element_blank() 
    ) +
    annotation_custom(leg1Grob, xmin=1, xmax=1, ymin=.95, ymax=1.3) +
    annotation_custom(leg2Grob, xmin=.6, xmax=.8, ymin=.75, ymax=1) 

out <- arrangeGrob(noleg, legs, ncol=2, widths=c(.85, .15))
print(out)