Question

I have the following data frame

z x y
1 1 a
2 2 a
3 1 a
4 2 a
5 1 b
6 9 b
7 9 b
8 8 b
9 7 b

when I do

p = ggplot(z,aes(x,group=y)) + geom_histogram(aes(y = ..density..,group=y)) +       facet_grid(y ~ .)
p

I get the faceted plots, but not with the percentages on the y-axis for each symbol within z$y.

Basically, I want a histogram chart, but with the percentages that show the frequency distribution within each value of z$y i.e. a,b.

In this case, under ‘a’, 50% is 1 and 50% is 2, and under ‘b’, 20% is 1, 40% is 9, 20% is 7 and 20% is 8. I want this charted as histograms using faceting.

Answer 1

That is not a histogram (there is no density estimation), but a bar chart.

d <- data.frame( 
  value = c(1,2,1,2,1,9,9,8),
  group = c(rep("a",4),rep("b",4))
)
# With counts
ggplot(d) + geom_bar(aes(factor(value))) + facet_grid(group ~ .)
# With percentages
ggplot(d) + 
  geom_bar(aes(factor(value), (..count..)/sum(..count..))) + 
  scale_y_continuous(formatter = 'percent') + 
  facet_grid(group ~ .)

Note: In more recent versions of ggplot2 we would use scale_y_continuous(labels = percent_format()) instead, and make sure to load the scales package.