ggplot2: how to change the order of a legend

Question

library(tidyverse)
library(ggplot2)
library(ggstance)
dat <- structure(list(Gender = structure(c(1L, 1L, 1L, 1L, 1L, 1L, 1L, 
                                           1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L), .Label = c("Female", "Male"
                                           ), class = "factor"), Status = c("Case", "Case", "Case", "Case", 
                                                                            "Control", "Control", "Control", "Control", "Case", "Case", "Case", 
                                                                            "Case", "Control", "Control", "Control", "Control"), Type = c("Age30", 
                                                                                                                                          "Age30", "Age30", "Age30", "Age30", "Age30", "Age30", "Age30", 
                                                                                                                                          "Age50", "Age50", "Age50", "Age50", "Age50", "Age50", "Age50", 
                                                                                                                                          "Age50"), Risk = c(21.59862, 3.27479, 1.10073, 1.70754, 8.85253, 
                                                                                                                                                             1.66318, 0.23228, 0.44844, 18.01182, 3.80135, 1.40662, 2.75944, 
                                                                                                                                                             4.81212, 1.93184, 0.29695, 0.72521), Disease = c("D1", "D2", 
                                                                                                                                                                                                              "D3", "D4", "D1", "D2", "D3", "D4", "D1", "D2", "D3", "D4", "D1", 
                                                                                                                                                                                                              "D2", "D3", "D4")), .Names = c("Gender", "Status", "Type", "Risk", 
                                                                                                                                                                                                                                             "Disease"), row.names = c(NA, -16L), class = "data.frame")

ggplot(dat, aes(x = Risk, y = forcats::fct_reorder(Status, Risk), 
                           group = Status,
                           fill = Type)) +
  facet_grid(Disease ~ ., switch = 'y') +
  geom_barh(aes(fill = interaction(Status, Type)), 
            stat = 'identity', position = 'stackv', color = "darkgrey",
            size = 0.3) +
  scale_x_continuous(limits = c(0, 100)) +
  labs( y = "Disease", x = "Risk",
       plot.title = element_text(hjust = 0.5)) + 
  theme_minimal() + 
  theme(
    text = element_text(size=10),
    strip.text.y = element_text(angle = 0),
    axis.text.y=element_blank(),
    axis.ticks.y=element_blank(),
    plot.title = element_text(size = 11, hjust = 0.5),
    legend.position = "bottom",
    legend.key.size = unit(0.2, "cm")) +
  scale_fill_manual("", 
                    values = c("#756bb1", "#2ca25f", "#bcbddc",
                                "#99d8c9"),
                    labels = c("Age 30 Case", "Age 30 Control", 
                               "Age 50 Case", "Age 50 Control"))

I would like to change the order of my legend labels to the following (from left to right):

[ ] Age 30 Case  [ ] Age 50 Case  [ ] Age 30 Control  [ ] Age 50 Control

I've tried reordering the values and labels but then the colors of the bars are then shuffled. I want the keep the colors as they are, ie dark purple corresponds to age 30 case, light purple to age 50 case. Dark green corresponds to age 30 control, and light green to age 50 control.

I understand that the colors correspond to the levels of the variable, but in my plot, the colors depend on the combination of 2 variables. So I couldn't think of a way to use relevel and change the order of my legend labels.

Answer 1

In your case, you want to set the breaks. The values and labels won't change the order. They should be given in the order of the breaks. The interaction() creates levels for you by putting a period between the two categories. You can check that with with(dat, levels(interaction(Status, Type))) . Then you can five an order for this via breaks= . For example

scale_fill_manual("", 
                breaks = c("Case.Age30", "Case.Age50",
                           "Control.Age30", "Control.Age50"),
                values = c("#756bb1", "#2ca25f", 
                           "#bcbddc", "#99d8c9"), 
                labels = c("Age 30 Case", "Age 50 Case",
                           "Age 30 Control", "Age 50 Control"))

Answer 2

This works better for me:

scale_fill_manual(
  values = c("blue", "lightgreen",  "#ecffb3", "red", "white"),
  breaks = c("Case.Age30", "Case.Age50", "Control.Age30", "Control.Age50", NA),
  na.value = "white",
  name = "Name of Legend:",
   limits = c("Age 30 Case", "Age 50 Case", "Age 30 Control", "Age 50 Control", NA)
)
# Note: Remove "white" from values, and NAs from breaks and limits if you don't want to plot NAs.