Free colour scales in facet_grid

Question

Say I have the following data frame:

# Set seed for RNG
set.seed(33550336)

# Create toy data frame
loc_x <- c(a = 1, b = 2, c = 3)
loc_y <- c(a = 3, b = 2, c = 1)
scaling <- c(temp = 100, sal = 10, chl = 1) 

df <- expand.grid(loc_name = letters[1:3], 
                  variables = c("temp", "sal", "chl"), 
                  season = c("spring", "autumn")) %>% 
  mutate(loc_x = loc_x[loc_name],
         loc_y = loc_y[loc_name],
         value = runif(nrow(.)),
         value = value * scaling[variables])

which looks like,

# > head(df)
#   loc_name variables season loc_x loc_y     value
# 1        a      temp spring     1     3 86.364697
# 2        b      temp spring     2     2 35.222573
# 3        c      temp spring     3     1 52.574082
# 4        a       sal spring     1     3  0.667227
# 5        b       sal spring     2     2  3.751383
# 6        c       sal spring     3     1  9.197086

I want to plot these data in a facet grid using variables and season to define panels, like this:

g <- ggplot(df) + geom_point(aes(x = loc_name, y = value), size = 5)
g <- g + facet_grid(variables ~ season) 
g

As you can see, different variables have very different scales. So, I use scales = "free" to account for this.

g <- ggplot(df) + geom_point(aes(x = loc_name, y = value), size = 5)
g <- g + facet_grid(variables ~ season, scales = "free") 
g

Mucho convenient. Now, say I want to do this, but plot the points by loc_x and loc_y and have value represented by colour instead of y position:

g <- ggplot(df) + geom_point(aes(x = loc_x, y = loc_y, colour = value), 
                             size = 5)
g <- g + facet_grid(variables ~ season, scales = "free") 
g <- g + scale_colour_gradient2(low = "#3366CC", 
                                mid = "white", 
                                high = "#FF3300", 
                                midpoint = 50) 
g

Notice that the colour scales are not free and, like the first figure, values for sal and chl cannot be read easily.

My question: is it possible to do an equivalent of scales = "free" but for colour, so that each row (in this case) has a separate colour bar? Or, do I have to plot each variable (i.e., row in the figure) and patch them together using something like cowplot?

In short, no, there is no free scales option for any other scale than x and y. If you want separate color bars you'll indeed need to make separate plots. Or rescale your colors within each facet.. — Axeman, Jan 02 '19 at 17:34

score 14 · Accepted Answer · answered Jan 02 '19 at 18:00

14

Using the development version of dplyr:

library(dplyr)
library(purrr)
library(ggplot2)
library(cowplot)

df %>% 
  group_split(variables, season) %>% 
  map(
    ~ggplot(., aes(loc_x, loc_y, color = value)) + 
      geom_point(size = 5) +
      scale_colour_gradient2(
        low = "#3366CC", 
        mid = "white", 
        high = "#FF3300", 
        midpoint = median(.$value)
      ) +
      facet_grid(~ variables + season, labeller = function(x) label_value(x, multi_line = FALSE))
  ) %>% 
  plot_grid(plotlist = ., align = 'hv', ncol = 2)

answered Jan 02 '19 at 18:00

Axeman

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That's a neat solution. Thanks! Is there a simple way of having each row on the same scale or would I have to have a different scale for each plot? By the way, I found that `ggarrange` from the `egg` package lines up plots better when the legend is of variable width due to labels of different lengths (e.g., cf `chl, spring` and `sal, spring`). – Dan Jan 02 '19 at 18:21
1

Oh right, I misread that you wanted it by row. Use `group_split(variables)` instead, and set `ncol` to 1 in `plot_grid`. You'll get three sets, with two facets each. Yes, there is different options for combining the plots! – Axeman Jan 02 '19 at 18:39

Free colour scales in facet_grid

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