ggplot geom_bar vs geom_histogram

Question

What is the difference (if any) between geom_bar and geom_histogram in ggplot? They seem to produce the same plot and take the same parameters.

Answer 1

• Bar charts provide a visual presentation of categorical data. Examples:
  • The number of people with red, black and brown hair
  • Look at the geom_bar help file. The examples are all counts.
  • Wikipedia page
• Histograms are used to plot density of interval (usually numeric) data. Examples,
  • Distributions of age and height
  • geom_hist help file. The examples are distribution of movie ratings.

ggplot2

After a bit more investigating, I think in ggplot2 there is no difference between geom_bar and geom_histogram. From the docs:

 geom_histogram(mapping = NULL, data = NULL, stat = "bin",
    position = "stack", ...)
 geom_bar(mapping = NULL, data = NULL, stat = "bin",
    position = "stack", ...)

I realise that in the geom_histogram docs it states:

geom_histogram is an alias for geom_bar plus stat_bin

but to be honest, I'm not really sure what this means, since my understanding of ggplot2 is that both stat_bin and geom_bar are layers (with a slightly different emphasis).

Answer 2

The default behavior is the same from both geom_bar and geom_histogram. This is because (and as @csgillespie mentioned), there is an implied stat_bin when you call geom_histogarm (understandable), and it is also the default statistics transformation applied to geom_bar (arguable behavior IMO). That's why you need to specify stat='identity' when you want the to plot the data as is.

The stat='bin' or stat_bin() is a statistical transformation that ggplot does for you. It provides you as output the variables surrounded with two dots (the ..count.. and ..density... If you don't specify stat='bin' you won't get those variables.