How can I plot a mean line by group in ggplot2?

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You can use the following basic syntax to plot a mean line by group in ggplot2:

#calculate mean points value by team
mean_team <- df %>% group_by(team) %>% summarise(mean_pts=mean(points))
 
#create scatterplot of assists vs points with mean line of points by team
ggplot(df, aes(x=assists, y=points)) +
  geom_point(aes(color=team)) +
  geom_hline(data=mean_team, aes(yintercept=mean_pts, col=team))

This particular example creates a scatterplot of the variables assists vs. points, then adds a line to represent the mean points value grouped by the team variable.

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

Example: Plot Mean Line by Group in ggplot2

Suppose we have the following data frame in R that contains information about points and assists for basketball players on three different teams:

#create data frame
df <- data.frame(team=rep(c('A', 'B', 'C'), each=5),
                 assists=c(2, 4, 4, 5, 6, 6, 7, 7,
                           8, 9, 7, 8, 13, 14, 12),
                 points=c(8, 8, 9, 9, 10, 9, 12, 13,
                          14, 15, 14, 14, 16, 19, 22))

#view data frame
df

   team assists points
1     A       2      8
2     A       4      8
3     A       4      9
4     A       5      9
5     A       6     10
6     B       6      9
7     B       7     12
8     B       7     13
9     B       8     14
10    B       9     15
11    C       7     14
12    C       8     14
13    C      13     16
14    C      14     19
15    C      12     22

We can use the following code to create a scatterplot of the variables assists vs. points, then add a line to represent the mean points value grouped by the team variable.

library(dplyr)
library(ggplot2)

#calculate mean points value by team
mean_team <- df %>% group_by(team) %>% summarise(mean_pts=mean(points))
 
#create scatterplot of assists vs points with mean line of points by team
ggplot(df, aes(x=assists, y=points)) +
  geom_point(aes(color=team)) +
  geom_hline(data=mean_team, aes(yintercept=mean_pts, col=team))

The three lines are color-coded to display the mean points value for each team.

We can display the mean_team data frame we created to see the actual mean points values for each team:

#view mean points value by team
mean_team

`summarise()` ungrouping output (override with `.groups` argument)
# A tibble: 3 x 2
  team  mean_pts
      
1 A          8.8
2 B         12.6
3 C         17  

From the output we can see:

• The mean points value for players on team A is 8.8
• The mean points value for players on team B is 12.6
• The mean points value for players on team C is 17

These values match the locations of the lines on the y-axis of the scatterplot that we created.