ggplot2: Controlling the order in fill option

Question

I have the following content in mydata:

Class       Category
"One"       "A"
"One"       "A"
"Two"       "A"
"Two"       "A"
"Three"     "B"
"Three"     "B"
"One"       "C"
"Two"       "C"

I use ggplot2:

ggplot(mydata) +
  aes(x = Category, fill = Class) +
  geom_bar() 

I get this result:

I notice that the "Class" items appear alphabetically. But I want the option to order them as follows:

1. ad hoc, so choose exact order
2. In order of appearance in data, so in this case, One, Two, Three
3. In reverse order of appearance in data: Three, Two, One

Answers gratefully appreciated.

Clarification

In case of doubt, here is the full working example of the above data:

Class <- c("One", "One", "Two", "Two", "Three", "Three", "One", "Two", "Four")
Category <- c("A", "A", "A", "A", "B", "B", "C", "C", "C")

mydata <-  data.frame(Class, Category)

ggplot(mydata) +
  aes(x = Category, fill = Class) +
  geom_bar() 

The Class key that is generated to the right is in the order:

Four, One, Three, Two

I want to have control over the order of the items in the key produced. (The colors are less important.)

Answer 1

You can specify the order of legend items with the the breaks parameter in scale_fill_discrete():

p <- ggplot(mydata) +
  aes(x = Category, fill = Class) +
  geom_bar() 

p + scale_fill_discrete(breaks = c("One", "Two", "Three", "Four"))
p + scale_fill_discrete(breaks = c("Four", "Three", "Two", "One"))

This leaves the underlying data & colour assignments unchanged.

Edit: To change the column stack order, you can assign the factor levels for Class before plotting it. Note that if you take this option, you won't need to manually specify the breaks again for the legend, as they'll follow the factor levels by default.

library(dplyr)

# alternative 1: does not change the underlying data frame
ggplot(mydata %>%
         mutate(Class = factor(Class,
                               levels = c("One", "Two", "Three", "Four")))) +
  aes(x = Category, fill = Class) +
  geom_bar() 

# alternative 2: changes the underlying data frame    
mydata2 <- mydata %>%
  mutate(Class = factor(Class,
                        levels = c("One", "Two", "Three", "Four")))
ggplot(mydata2) +
  aes(x = Category, fill = Class) +
  geom_bar() 

Answer 2

Let's say you want the order to be Three, Two, One, so you need to use:

setattr(mydata$Class,"levels",c("Three","Two","One"))

First and then run your ggplot code. Please mark this as the correct answer if you are satisfied with the solution. thanks :)