add custom r2dii color scales

[cjyetman]

Now that #19 adds a custom ggplot theme, let's also add color scales to be able to easily use the 2dii color palette. Something like...

library(ggplot2)
library(r2dii.ggplot)


r2dii_pal <- function() {
  values <- r2dii_palette_colours()$colour_hex
  max_n <- length(values)
  f <- scales:::manual_pal(values)
  attr(f, "max_n") <- max_n
  f
}

scale_colour_r2dii <- function(...) {
  discrete_scale("colour", "r2dii", r2dii_pal(), ...)
}

scale_color_r2dii <- scale_colour_r2dii

scale_fill_r2dii <- function(...) {
  discrete_scale("fill", "r2dii", r2dii_pal(), ...)
}


scatter <- 
  ggplot(mtcars) +
  geom_point(aes(x = wt, y = mpg, colour = factor(gear))) +
  facet_wrap(~am) +
  ggtitle("Diamond Prices")

scatter_r2dii <- scatter + theme_2dii_ggplot() + scale_color_r2dii()

bar <- ggplot(mtcars, aes(factor(cyl), fill = factor(vs))) + geom_bar()
bar_r2dii <- bar + theme_2dii_ggplot() + scale_fill_r2dii()

gridExtra::grid.arrange(scatter, scatter_r2dii, bar, bar_r2dii, nrow = 2)