How to add different horizontal lines to different facets

Hi all,

I have a question about adding separate horizontal lines to different panels in ggplot bar charts.
For example, the dataframe is like this:

>     value      group   level
>   -3.659497     1     Level 1
>   -2.372006     2     Level 1
>   -3.435326     3     Level 1
>   -3.492647     4     Level 1
>   -3.589319     5     Level 1
>   -5.498233     1     Level 2
>   -2.720646     2     Level 2
>   -5.055747     3     Level 2
>   -5.113817     4     Level 2
>   -5.311411     5     Level 2

I draw bar plots and combine them together using the code below:

> ggplot(a,aes(group,value))+
>   geom_bar(position='dodge',stat='identity')+ facet_wrap(~level,ncol=1)+
>   geom_hline(yintercept=mean(a$value))
> print(

However, I want to add the mean value of Level 1 and Level 2 on the two panels (Level 1 and Level 2) separately, rather than using a whole mean value as the horizontal lines in each panel. How could I achieve this? Thanks for your help.

You can create separate data.frame to calculate means then assign them to geom_hline() layer.

a_mean <- a %>% 
  group_by(level) %>% 
  summarize(mean_val = mean(value))
# A tibble: 2 x 2
  level   mean_val
  <chr>      <dbl>
1 Level 1    -3.31
2 Level 2    -4.74
  geom_bar(position='dodge',stat='identity')+ facet_wrap(~level,ncol=1)+
  geom_hline(data= a_mean, aes(yintercept=mean_val))


Thanks, I tried to install "tidyverse" package in order to use the pipe function, but got the error below. What is the problem of this? Thanks.

Loading required package: tidyverse
Error: package or namespace load failed for ‘tidyverse’ in dyn.load(file, DLLpath = DLLpath, ...):
unable to load shared object '/Library/Frameworks/R.framework/Versions/3.4/Resources/library/readxl/libs/':
dlopen(/Library/Frameworks/R.framework/Versions/3.4/Resources/library/readxl/libs/, 6): Library not loaded: /Library/Frameworks/R.framework/Versions/3.5/Resources/lib/libR.dylib
Referenced from: /Library/Frameworks/R.framework/Versions/3.4/Resources/library/readxl/libs/
Reason: Incompatible library version: requires version 3.5.0 or later, but libR.dylib provides version 3.4.0

If you want to install tidyverse use install.packages("tidyverse") but maybe you want to do an update first update.packages(checkBuilt=TRUE, ask=FALSE)

Just remove the a$ in the hline code.
Ggplot inherits the data. But by stating a$ you pass the whole dataframe and take the average

Thanks, it works well in this case.
If it is for monthly data, and each group represent one month. I use mean value of monthly data as the line. So each month will have a different number, how to plot such case and connect the mean value from each month onto this figure? Thanks again.

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