Histograms with variable size binwidth in ggplot2

Question

I have the following data

 dati <- read.table(text="
        class     num
    1     0.0   63530
    2     2.5   27061
    3     3.5   29938
    4     4.5   33076
    5     5.6   45759
    6     6.5   72794
    7     8.0  153177
    8    10.8  362124
    9    13.5  551051
    10   15.5  198634
  ")

And I want to produce a histogram with variable size bins, so that the area of each bar reflects the total numerosity (num) of each bin. I tried

bins <- c(0,4,8,11,16)
p <- ggplot(dati) +
  geom_histogram(aes(x=class,weight=num),breaks = bins)

however, this produces a histogram where the length of each bar is equal to total numerosity of each bin. Because bin widths are variable, areas are not proportional to numerosity. I could not solve this apparently easy problem within ggplot2. Can anyone help me?

Answer 1

I think you are looking for a density plot - this closely related question has most of the answer. You call y = ..density.. in geom_histogram().

This works because stat_bin (recall geom_histogram() is geom_bar() + stat_bin(), and stat_bin() constructs a data frame with columns count and density. Thus calling y = ..density.. pulls the right column for density, whereas the default (counts) is as if you call y = ..count...

##OP's code
ggplot(dati) +  geom_histogram(aes(x=class, weight=num),
 breaks = bins)

##new code (density plot)
ggplot(dati) +  geom_histogram( aes(x=class,y = ..density.., weight=num),
 breaks = bins, position = "identity")

You can find some further examples in the online ggplot2 help page for geom_histogram().

Answer 2

It sounds to me like you asking for how to produce variable sized bar widths. If so, you just need to call the 'width' parameter in your ggplot aesthetics like this:

ggplot(data, aes(x = x, y = y, width = num))

this method is discussed more in the following question: Variable width bars in ggplot2 barplot in R