如何在R中使用ggplot()绘制符合需求的堆叠柱状图？

Hey there! Let's get that stacked bar chart working for you. The issue with your original code is twofold: you're calling geom_bar() multiple times (which doesn't work for stacking), and ggplot2 relies on tidy (long-format) data for this type of plot—your current dataset is probably in wide format (with separate columns for Absent and Present). Here's a step-by-step solution:

1. Convert Your Data to Long Format #

First, we'll reshape your wide data into long format using the pivot_longer() function from the tidyr package. This creates a single column for attendance status (Absent/Present) and another for the corresponding counts.

# Load required packages
library(ggplot2)
library(tidyr)

# Example wide-format data (replace with your actual data)
out <- data.frame(
  Session = c("S1", "S2", "S3"),
  Absent = c(5, 3, 7),
  Present = c(15, 17, 13)
)

# Reshape to long format
out_long <- out %>%
  pivot_longer(
    cols = c(Absent, Present),  # Columns to reshape
    names_to = "Attendance_Status",  # Name of the new category column
    values_to = "Count"  # Name of the new value column
  )

2. Create the Stacked Bar Chart #

Now we can use the long-format data to build the stacked bar chart. The key is to map x to Session, y to Count, and fill to your attendance status column. We use stat = "identity" because we're using pre-calculated counts (not letting ggplot count observations automatically).

# Build the plot
p <- ggplot(out_long, aes(x = Session, y = Count, fill = Attendance_Status)) +
  geom_bar(stat = "identity") +  # Use identity since we have raw counts
  # Add labels for clarity
  labs(
    title = "Attendance by Session",
    x = "Session",
    y = "Number of Participants",
    fill = "Status"
  ) +
  # Optional: Use a clean theme
  theme_minimal()

# Display the plot
print(p)

Why Your Original Code Failed #

• Multiple geom_bar() calls don't stack values—they just layer bars on top of each other, which leads to incorrect visualization.
• You didn't map the y axis to your count values, and the fill aesthetic wasn't properly linked to the two attendance categories.
• Without stat = "identity", geom_bar() defaults to counting rows, which isn't what you want when you have pre-defined counts in Absent and Present.

If your data is already in long format, you can skip the pivot_longer() step and jump straight to the ggplot code!

内容的提问来源于stack exchange，提问作者Manoj