r - Assign color to 2 different geoms and get 2 different legends

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"))