You can use the following basic syntax to order the items on the x-axis of a plot in ggplot2:

ggplot(df, aes(x=factor(x_var, level=c('value1', 'value2', 'value3')), y=y_var)) +
  geom_col()

The following examples show how to use this syntax in practice.

Example: Order Items on x-axis in ggplot2

Suppose we have the following data frame in R that shows the points scored by various basketball teams:

#create data frame
df <- data.frame(team=c('Mavs', 'Heat', 'Nets', 'Lakers'),
                 points=c(100, 122, 104, 109))

#view data frame
df

    team points
1   Mavs    100
2   Heat    122
3   Nets    104
4 Lakers    109

If we create a bar plot to visualize the points scored by each team, ggplot2 will automatically order the bars in alphabetical order:

library(ggplot2)

#create bar plot
ggplot(df, aes(x=team, y=points)) +
  geom_col()

To specify an order for the bars on the x-axis, we can use the level argument as follows:

library(ggplot2)

#create bar plot with specific axis order
ggplot(df, aes(x=factor(team, level=c('Mavs', 'Heat', 'Nets', 'Lakers')), y=points)) +
  geom_col()

The bars are now in the exact order that we specified inside the level argument.

You may also want to use xlab() to rename the x-axis to something that is easier to read:

library(ggplot2)

#create bar plot with specific axis order
ggplot(df, aes(x=factor(team, level=c('Mavs', 'Heat', 'Nets', 'Lakers')), y=points)) +
  geom_col() +
  xlab('Team')