# Using palettes with ggplot2

This vignette shows you how to use palettes as colour and fill scales with ggplot2.

    library(palettes)
    library(ggplot2)
    library(scales)

## Colour scales

For discrete colours use `scale_colour_palette_d()`.

    ggplot(diamonds[sample(nrow(diamonds), 1000), ], aes(carat, price)) +
      geom_point(aes(colour = clarity)) +
      scale_colour_palette_d(met_palettes$VanGogh3)

Continuous or binned colours can be used with continuous data. For continuous colours use `scale_colour_palette_c()`. For binned colours use `scale_colour_palette_b()`.

    hwy_mpg <- ggplot(mpg, aes(displ, hwy, colour = hwy)) +
      geom_point()

    hwy_mpg + scale_colour_palette_c(met_palettes$Greek)

    hwy_mpg + scale_colour_palette_b(met_palettes$Greek)

## Fill scales

For discrete fills use `scale_fill_palette_d()`.

    ggplot(diamonds, aes(x = price, fill = cut)) +
      geom_histogram(position = "dodge", binwidth = 1000) +
      scale_fill_palette_d(pnw_palettes$Sunset)

Continuous or binned fills can be used with continuous data. For continuous fills use `scale_fill_palette_c()`. For binned fills use `scale_fill_palette_b()`.

    eruptions <- ggplot(faithfuld, aes(waiting, eruptions, fill = density)) +
      geom_tile()

    eruptions + scale_fill_palette_c(viridis_palettes$mako)

    eruptions + scale_fill_palette_b(viridis_palettes$mako)

## Customizing scales

The `scale_` functions can all pass arguments to the appropriate scale constructors in ggplot2 if you need to control the name, limits, breaks, labels, and so forth of colour and fill scales. The arguments are passed with `...` to the appropriate scale constructor for each function:

- `ggplot2::discrete_scale()` for `scale_colour_palette_d()` and `scale_fill_palette_d()`
- `ggplot2::continuous_scale()` for `scale_colour_palette_c()` and `scale_fill_palette_c()`
- `ggplot2::binned_scale()` for `scale_colour_palette_b()` and `scale_fill_palette_b()`

This is particularly useful when you need to change how colours are mapped to values in a scale. For example, a common problem when working with divergent colour palettes is getting the middle colour in a palette to align with a fixed value representing the mid-point of a continuous scale.

To demonstrate, say we have mean treatment outcome results from four groups in a clinical trial.

    treatment_results <- data.frame(
      group   = LETTERS[1:4],
      outcome = c(-0.5, -1.1, 1.9, 2.3)
    )

We want to plot the outcomes as a diverging bar chart with divergent fills. By default, the fill scale’s mid-point is the mean of the four groups, but we want it to be zero.

    ggplot(treatment_results, aes(x = group, y = outcome, fill = outcome)) +
      geom_col() +
      scale_fill_palette_c(met_palettes$Benedictus)

We can use the `rescaler` argument in `ggplot2::continuous_scale()`, which accepts a function used to scale the input values to the range [0, 1], to scale the fill values to have a mid-point of zero. For scaling the mid-point use `scales::rescale_mid()`.

    ggplot(treatment_results, aes(x = group, y = outcome, fill = outcome)) +
      geom_col() +
      scale_fill_palette_c(
        met_palettes$Benedictus,
        rescaler = ~ rescale_mid(.x, mid = 0)
      )