Every Geom Has a Stat

This is one of the most under-appreciated ideas in ggplot2, and grasping it will make a lot of "magic" behavior suddenly make sense. Every layer runs a statistical transformation before it draws. Sometimes that transformation does nothing; sometimes it does a lot.

The mystery of geom_bar

Recall that geom_bar() needs only an x aesthetic, yet it produces bars with heights. Where do the heights come from? You never supplied a y.

The answer: geom_bar() has a statistic attached — stat_count — that runs first. It groups the rows by class, counts them, and produces a new computed variable, count. That becomes the bar height. The geom draws bars; the stat invented the y-values.

The layer pipeline

Every layer is really data → stat → geom. The stat sits between your raw data and the drawn marks:

For a scatter plot the stat is stat_identity — it passes the data through unchanged, which is why geom_point() needs you to supply both x and y. For a histogram the stat is stat_bin — it slices x into bins and counts each. The geom is the same idea as always (draw marks); the stat is what differs.

Geoms and stats are paired — but separable

Each geom ships with a default stat, and each stat ships with a default geom. They are two sides of one layer:

Because they are separable, you can override the stat. Telling geom_bar() to use stat = "identity" makes it stop counting and use your y directly — which is exactly what geom_col() does under the hood:

Computed variables: the after-stat values

A stat produces new columns you did not have. stat_count produces count and prop; stat_bin produces count and density. You can use these computed variables in mappings via after_stat(). For example, to make a histogram show density instead of raw counts on the y-axis:

This is advanced, but the point is conceptual: the stat creates new variables, and you can reach into them. The default histogram maps y = after_stat(count) for you; here we asked for density instead.

QuestionSelect one

geom_bar() is given only an x aesthetic, yet it draws bars with heights. Where do the heights come from?

ggplot2 picks random heights.

You must have secretly mapped y.

geom_bar()'s default statistic, stat_count, counts the rows in each x category and supplies that count as the bar height.

The heights are always 1.

QuestionSelect one

What is the relationship between geom_col() and geom_bar(stat = "identity")?

They are unrelated; geom_col uses a special coordinate system.

geom_col() counts rows while geom_bar(stat = "identity") does not.

They are equivalent: both use stat = "identity", meaning the data passes through unchanged and the supplied y becomes the bar height.

geom_bar(stat = "identity") is invalid syntax.

QuestionSelect one

Why does ggplot2 separate the statistic from the geom within a layer?

To make plots render more slowly but more accurately.

Because each geom can only ever use one specific stat.

So the computation (e.g. counting, binning, smoothing) is independent of the drawing (bars, points, lines), letting any stat pair with different geoms.

Purely for historical reasons with no practical effect.

Key takeaways

• Every layer runs a stat before drawing: data → stat → geom.
• geom_point() uses stat_identity (no change); geom_bar() uses stat_count; geom_histogram() uses stat_bin; and so on.
• Stats create new computed variables (like count, density) that become aesthetics; reach them with after_stat().
• Stat and geom are separable — overriding the stat is how geom_col() equals geom_bar(stat = "identity").