Issue with ggplot2, geom_bar, and position="dodge": stacked has correct y values, dodged does not

Question

I'm having quite the time understanding `geom_bar()` and `position="dodge"`. I was trying to make some bar graphs illustrating two groups. Originally the data was from two separate data frames. Per this question, I put my data in long format. My example: ``` test <- data.frame(names=rep(c("A","B","C"), 5), values=1:15) test2 <- data.frame(names=c("A","B","C"), values=5:7) df <- data.frame(names=c(paste(test$names), paste(test2$names)), num=c(rep(1, nrow(test)), rep(2, nrow(test2))), values=c(test$values, test2$values)) ``` I use that example as it's similar to the spend vs. budget example. Spending has many rows per `names` factor level whereas the budget only has one (one budget amount per category). For a stacked bar plot, this works great: ``` ggplot(df, aes(x=factor(names), y=values, fill=factor(num))) + geom_bar(stat="identity") ``` In particular, note the y value maxes. They are the sums of the data from `test` with the values of `test2` shown on blue on top. Based on other ques…

Accepted Answer

I think the problem is that you want to stack within values of the `num` group, and dodge between values of `num`. It might help to look at what happens when you add an outline to the bars.

```
library(ggplot2)
set.seed(123)
df <- data.frame(
  id     = 1:18,
  names  = rep(LETTERS[1:3], 6),
  num    = c(rep(1, 15), rep(2, 3)),
  values = sample(1:10, 18, replace=TRUE)
)
```

By default, there are a lot of bars stacked - you just don't see that they're separate unless you have an outline:

```
# Stacked bars
ggplot(df, aes(x=factor(names), y=values, fill=factor(num))) + 
  geom_bar(stat="identity", colour="black")
```

If you dodge, you get bars that are dodged between values of `num`, but there may be multiple bars within each value of `num`:

```
# Dodged on 'num', but some overplotted bars
ggplot(df, aes(x=factor(names), y=values, fill=factor(num))) + 
  geom_bar(stat="identity", colour="black", position="dodge", alpha=0.1)
```

If you also add `id` as a grouping var, it'll dodge all of them:

```
# Dodging with unique 'id' as the grouping var
ggplot(df, aes(x=factor(names), y=values, fill=factor(num), group=factor(id))) + 
  geom_bar(stat="identity", colour="black", position="dodge", alpha=0.1)
```

I think what you want is to both dodge and stack, but you can't do both. So the best thing is to summarize the data yourself.

```
library(plyr)
df2 <- ddply(df, c("names", "num"), summarise, values = sum(values))

ggplot(df2, aes(x=factor(names), y=values, fill=factor(num))) + 
  geom_bar(stat="identity", colour="black", position="dodge")
```

Issue with ggplot2, geom_bar, and position="dodge": stacked has correct y values, dodged does not

Solution

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