如何将图例添加到 ggplot 多个 geom_bar

Question

I have a code like this

# two data frames
sf =  data.frame(days = c('05-06', '05,08'), Frequency = c(0, 2))
normal = data.frame(days = c('05-06', '05,07'), Frequency = c(1, 3))

# plot
ggplot() + 
  geom_bar(aes(y = Frequency, x = days), data = normal, 
           stat="identity", colour = 'black') +
  geom_bar(aes(y = Frequency, x = days), data = sf, 
           stat="identity", colour = 'red')

I want to add legend to it with red and black color, I tried this one which is the most related solution to my question, but it just shows the legend for the first barplot. How can I have a barplot legend with black and red color?

Notice: it is not one data frame, they are two separate data frames, with different number of rows, so I can not merge them together. So I plot two bar plots. The above code fill the plots, what I want is to add a legend that shows the first bar plot is black and the second one is red.

Answer 1

As with many ggplot questions, the problem you are facing stems from the data not being in the correct format. You should have a single dataframe, with the columns days , Frequency and group .

Set the colour within the aesthetic group of the ggplot, and then specify an manual colour scale to set the colours and black.

library(ggplot2)

df =  data.frame(days = c('05-06', '05,08', '05-06', '05,07'),
                 Frequency = c(0, 2, 1, 3),
                 group = c("sf", "sf", "normal", "normal"))


# plot
ggplot(df, aes(y = Frequency, x = days, fill = group)) + 
  geom_bar(stat="identity") +
  scale_fill_manual(values = c("red", "black"))

I would encourage reading the basics of ggplot reference guidance to get a bit more of an understanding about how to use the data: http://ggplot2.tidyverse.org/reference/ . In addition, the ggplot cheatsheet is a useful reference: https://www.rstudio.com/wp-content/uploads/2015/03/ggplot2-cheatsheet.pdf