Grouping bars in a plot by column value using ggplot2 in R-CodePudding

I have count data of invertebrates along a transect line. The data includes 3 columns- one column for the date the data was collected on, one for the transect identification number, and one for the species observed.

structure(list(Date = c("8/22/2022", "8/22/2022", "8/23/2022", 
"8/23/2022", "8/23/2022", "8/23/2022", "8/23/2022", "8/23/2022", 
"8/23/2022", "8/23/2022", "8/23/2022", "8/23/2022", "8/23/2022", 
"8/23/2022", "8/23/2022", "8/18/2022", "8/18/2022", "8/18/2022", 
"8/18/2022", "8/18/2022", "8/18/2022", "8/18/2022", "8/18/2022", 
"8/18/2022", "8/18/2022", "8/18/2022", "8/18/2022", "8/18/2022", 
"8/23/2022", "8/23/2022", "8/23/2022", "8/23/2022", "8/23/2022", 
"8/23/2022", "8/23/2022", "8/23/2022", "8/23/2022", "8/23/2022", 
"8/23/2022", "8/23/2022", "8/22/2022", "8/22/2022", "8/22/2022", 
"8/22/2022", "8/22/2022", "8/22/2022", "8/22/2022", "8/22/2022", 
"8/22/2022", "8/22/2022", "8/22/2022", "8/22/2022", "8/22/2022", 
"8/22/2022", "8/22/2022", "8/22/2022", "8/22/2022", "8/22/2022", 
"8/22/2022", "8/22/2022", "8/22/2022", "8/22/2022", "8/22/2022", 
"8/22/2022", "8/18/2022", "8/18/2022", "8/18/2022", "8/18/2022", 
"8/18/2022", "8/18/2022", "8/18/2022", "8/18/2022", "8/18/2022", 
"8/16/2022", "8/16/2022", "8/16/2022", "8/16/2022", "8/16/2022", 
"8/16/2022", "8/16/2022", "8/16/2022", "8/16/2022", "8/16/2022", 
"8/16/2022", "8/16/2022", "8/16/2022", "8/16/2022", "8/16/2022", 
"8/16/2022", "8/16/2022", "8/16/2022", "8/16/2022", "8/16/2022", 
"8/16/2022", "8/16/2022", "8/16/2022"), Transect = c(8L, 8L, 
4L, 4L, 5L, 5L, 5L, 6L, 6L, 7L, 8L, 8L, 9L, 9L, 9L, 4L, 4L, 4L, 
5L, 5L, 6L, 6L, 7L, 8L, 8L, 9L, 9L, 9L, 1L, 1L, 1L, 1L, 2L, 2L, 
2L, 2L, 3L, 3L, 3L, 3L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 3L, 3L, 
3L, 3L, 4L, 4L, 5L, 5L, 6L, 6L, 7L, 7L, 8L, 9L, 9L, 9L, 1L, 1L, 
1L, 1L, 2L, 2L, 2L, 3L, 3L, 1L, 1L, 1L, 2L, 2L, 2L, 3L, 3L, 3L, 
4L, 4L, 4L, 5L, 6L, 7L, 7L, 7L, 8L, 8L, 8L, 9L, 9L, 9L), Species = c("RCRAB", 
"DOL", "DOL", "STAR", "DOL", "RCRAB", "STAR", "DOL", "LOB", "DOL", 
"DOL", "RCRAB", "DOL", "LOB", "STAR", "DOL", "LOB", "STAR", "DOL", 
"RCRAB", "DOL", "RCRAB", "DOL", "DOL", "RCRAB", "DOL", "STAR", 
"RCRAB", "DOL", "STAR", "RCRAB", "URCH", "DOL", "RCRAB", "URCH", 
"STAR", "DOL", "LOB", "STAR", "URCH", "DOL", "RCRAB", "STAR", 
"URCH", "STAR", "DOL", "URCH", "RCRAB", "DOL", "STAR", "URCH", 
"RCRAB", "DOL", "STAR", "DOL", "LOB", "DOL", "RCRAB", "DOL", 
"RCRAB", "DOL", "DOL", "STAR", "URCH", "DOL", "STAR", "RCRAB", 
"LOB", "DOL", "STAR", "RCRAB", "DOL", "LOB", "DOL", "STAR", "LOB", 
"DOL", "STAR", "URCH", "DOL", "STAR", "RCRAB", "DOL", "LOB", 
"STAR", "DOL", "DOL", "DOL", "RCRAB", "STAR", "STAR", "DOL", 
"RCRAB", "DOL", "STAR", "RCRAB")), class = "data.frame", row.names = c(NA, 
-96L))

I want to create a multiplot where each date has a separate plot, with the X value being the transect number and the Y value being the number of species found on that transect. So far, I have this:

library(ggplot2)
invertplot <- ggplot(data=invert, aes(Transect, Species))  
  geom_bar(stat='identity')  
  labs(title="Number of Invertebrate Species per Transect Steering Rocks August 2022",
       y="Number of Species",
       x="Transect Number")  
  facet_wrap(~Date)
invertplot

Which gives me a plot where each individual species is listed on the Y axis, and the X axis is the number of that species in the entire data set.

How do I get ggplot to group the values by transect number, and not species? Thanks in advance!

CodePudding user response：

If you want a bar plot that simply counts observations, don't use stat = "identity". The default behaviour of geom_bar is to use stat_count. So I think you just need:

library(tidyverse)

ggplot(data = invert, aes(factor(Transect)))  
  geom_bar()  
  labs(title = paste0("Number of Invertebrate Species per Transect ",
                      "Steering Rocks August 2022"),
       y = "Number of Species",
       x = "Transect Number")  
  facet_wrap( ~ Date)

In case there are multiple recordings of the same species on the same transect on the same date that you only want to count once, you would be safer to do:

ggplot(data = invert %>% count(Transect, Species, Date), aes(factor(Transect)))  
  geom_bar()  
  labs(title = paste0("Number of Invertebrate Species per Transect ",
                      "Steering Rocks August 2022"),
       y = "Number of Species",
       x = "Transect Number")  
  facet_wrap( ~ Date)

But this gives the same output for your current example data.