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My data is about boardgames which contain 1 to many catgeories each like this :

categorie1 categorie2 categorie3
  Deduction   Medieval  Word Game
  Deduction   Medieval  NA
  Card Game   Medieval  Zombies
     Horror   NA        NA
     Horror   Medieval  Zombies

I would like to create a barplot showing the most common categories accross games but i can't figure out how with multiple columns instead of one. Is there a dplyr method ?

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The function you are looking for is gather from the tidyr package. This function takes a wide data.frame and makes it a long data.frame. gather is easy to use:

library(dplyr)
library(tidyr)

# Building a sample data.frame like your example data.
df <- data.frame(Boardgame = c("Game1", "Game2", "Game3", "Game4", "Game5"),
             categorie1 = c("Deduction", "Deduction", "Card Game", "Horror", "Horror"),
             categorie2 = c("Medieval", "Medieval", "Medieval", NA, "Medieval"),
             categorie3 = c("Word Game", NA, "Zombies", NA, "Zombies"),
             stringsAsFactors = FALSE)

# Using gather from tidyr and some dplyr functions:
df %>%
  gather(key = "Cat_Label",    # Key is the name of the column that will holdthe old column names
         value = "Categorie",  # Value is the name of the column that will hold the data
         -Boardgame,           # Ignore the Boardgame column, use every other column
         na.rm = TRUE) %>%     # Remove NA values
  arrange(Boardgame) %>%       # Sort by Boardgame
  select(-Cat_Label)           # Remove the unneeded Cat_Label column (if you want)

# Results:
   Boardgame Categorie
1      Game1 Deduction
2      Game1  Medieval
3      Game1 Word Game
4      Game2 Deduction
5      Game2  Medieval
6      Game3 Card Game
7      Game3  Medieval
8      Game3   Zombies
9      Game4    Horror
10     Game5    Horror
11     Game5  Medieval
12     Game5   Zombies

The -boardgame notation in the gather function means that every column but Boardgame will be gathered, even if you have 200 categorie columns. Once you have your data.frame, you can use ggplot2 to visualize the Categorie column as you see fit.

It should be noted that development of gather by the Tidyverse team is complete. A new, more general function pivot_longer has been implemented to replace gather. It has similar usage in a simple case, but the arguments are a little different:

df %>% 
  pivot_longer(cols = -Boardgame,         # You now explicitly declare the columns
               names_to = "Cat_Label",    # New column containing old column names
               values_to = "Categorie",   # New column containing old column values
               values_drop_na = TRUE)     # Remove NA values
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