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

Jordan Woolfrey 0
Viewed 31

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. enter image description here

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

ggplot2 r
1 Answers

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)

enter image description here

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.

Allan Cameron0
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