Associate a color palette with ggplot2 theme

Question

I want my ggplot2 theme to use a specific set of colors, but don't see how to avoid a separate line outside of the theme.

I have this data:

library(ggplot2)
mycars <- mtcars
mycars$cyl <- as.factor(mycars$cyl)

And here's a dummy theme I plot with:

mytheme <- theme(panel.grid.major = element_line(size = 2))

ggplot(mycars, aes(x = wt, y = mpg)) +
  geom_point(aes(color = cyl)) +
  mytheme

I want the point colors to default to my custom palette:

mycolors <- c("deeppink", "chartreuse", "midnightblue")

Can I somehow add that to my ggplot2 theme so that I don't constantly repeat this extra line of code at the end:

ggplot(mycars, aes(x = wt, y = mpg)) +
  geom_point(aes(color = cyl)) +
  mytheme +
  scale_color_manual(values = mycolors)

I tried:

mytheme2 <- mytheme + scale_color_manual(values = mycolors)

But got:

Error: Don't know how to add scale_color_manual(values = mycolors) to a theme object

Answer 1

I sometimes use this little trick to change colour palettes between plots,

library(ggplot2)
mycars <- mtcars
mycars$cyl <- as.factor(mycars$cyl)

scale_colour_discrete <- function(...) scale_colour_manual(values=palette())

(p <- ggplot(mycars, aes(x = wt, y = mpg)) +
  geom_point(aes(color = cyl)) )

palette(c("deeppink", "chartreuse", "midnightblue"))
p

palette(RColorBrewer::brewer.pal(5, "Set1"))
p

Answer 2

Hi you can put your custom element in a list :

# Data
library("ggplot2")
mycars <- mtcars
mycars$cyl <- as.factor(mycars$cyl)

# Custom theme
mytheme <- theme(panel.grid.major = element_line(size = 2))
mycolors <- c("deeppink", "chartreuse", "midnightblue")
# put the elements in a list
mytheme2 <- list(mytheme, scale_color_manual(values = mycolors))

# plot 
ggplot(mycars, aes(x = wt, y = mpg)) +
  geom_point(aes(color = cyl)) +
  mytheme2