Extending ggplot2

This vignette documents the official extension mechanism provided in ggplot2 2.0.0. This vignette is a high-level adjunct to the low-level details found in ?Stat, ?Geom and ?theme. You’ll learn how to extend ggplot2 by creating a new stat, geom, or theme.

As you read this document, you’ll see many things that will make you scratch your head and wonder why on earth is it designed this way? Mostly it’s historical accident - I wasn’t a terribly good R programmer when I started writing ggplot2 and I made a lot of questionable decisions. We cleaned up as many of those issues as possible in the 2.0.0 release, but some fixes simply weren’t worth the effort.


All ggplot2 objects are built using the ggproto system of object oriented programming. This OO system is used only in one place: ggplot2. This is mostly historical accident: ggplot2 started off using proto because I needed mutable objects. This was well before the creation of (the briefly lived) mutatr, reference classes and R6: proto was the only game in town.

But why ggproto? Well when we turned to add an official extension mechanism to ggplot2, we found a major problem that caused problems when proto objects were extended in a different package (methods were evaluated in ggplot2, not the package where the extension was added). We tried converting to R6, but it was a poor fit for the needs of ggplot2. We could’ve modified proto, but that would’ve first involved understanding exactly how proto worked, and secondly making sure that the changes didn’t affect other users of proto.

It’s strange to say, but this is a case where inventing a new OO system was actually the right answer to the problem! Fortunately Winston is now very good at creating OO systems, so it only took him a day to come up with ggproto: it maintains all the features of proto that ggplot2 needs, while allowing cross package inheritance to work.

Here’s a quick demo of ggproto in action:

A <- ggproto("A", NULL,
  x = 1,
  inc = function(self) {
    self$x <- self$x + 1
#> [1] 1
#> [1] 2
#> [1] 4

The majority of ggplot2 classes are immutable and static: the methods neither use nor modify state in the class. They’re mostly used as a convenient way of bundling related methods together.

To create a new geom or stat, you will just create a new ggproto that inherits from Stat, Geom and override the methods described below.

Creating a new stat

The simplest stat

We’ll start by creating a very simple stat: one that gives the convex hull (the c hull) of a set of points. First we create a new ggproto object that inherits from Stat:

The two most important components are the compute_group() method (which does the computation), and the required_aes field, which lists which aesthetics must be present in order to for the stat to work.

Next we write a layer function. Unfortunately, due to an early design mistake I called these either stat_() or geom_(). A better decision would have been to call them layer_() functions: that’s a more accurate description because every layer involves a stat and a geom.

All layer functions follow the same form - you specify defaults in the function arguments and then call the layer() function, sending ... into the params argument. The arguments in ... will either be arguments for the geom (if you’re making a stat wrapper), arguments for the stat (if you’re making a geom wrapper), or aesthetics to be set. layer() takes care of teasing the different parameters apart and making sure they’re stored in the right place:

(Note that if you’re writing this in your own package, you’ll either need to call ggplot2::layer() explicitly, or import the layer() function into your package namespace.)

Once we have a layer function we can try our new stat:

(We’ll see later how to change the defaults of the geom so that you don’t need to specify fill = NA every time.)

Once we’ve written this basic object, ggplot2 gives a lot for free. For example, ggplot2 automatically preserves aesthetics that are constant within each group:

We can also override the default geom to display the convex hull in a different way:

Stat parameters

A more complex stat will do some computation. Let’s implement a simple version of geom_smooth() that adds a line of best fit to a plot. We create a StatLm that inherits from Stat and a layer function, stat_lm():

StatLm is inflexible because it has no parameters. We might want to allow the user to control the model formula and the number of points used to generate the grid. To do so, we add arguments to the compute_group() method and our wrapper function:

Note that we don’t have to explicitly include the new parameters in the arguments for the layer, ... will get passed to the right place anyway. But you’ll need to document them somewhere so the user knows about them. Here’s a brief example. Note @inheritParams ggplot2::stat_identity: that will automatically inherit documentation for all the parameters also defined for stat_identity().

stat_lm() must be exported if you want other people to use it. You could also consider exporting StatLm if you want people to extend the underlying object; this should be done with care.

Picking defaults

Sometimes you have calculations that should be performed once for the complete dataset, not once for each group. This is useful for picking sensible default values. For example, if we want to do a density estimate, it’s reasonable to pick one bandwidth for the whole plot. The following Stat creates a variation of the stat_density() that picks one bandwidth for all groups by choosing the mean of the “best” bandwidth for each group (I have no theoretical justification for this, but it doesn’t seem unreasonable).

To do this we override the setup_params() method. It’s passed the data and a list of params, and returns an updated list.

I recommend using NULL as a default value. If you pick important parameters automatically, it’s a good idea to message() to the user (and when printing a floating point parameter, using signif() to show only a few significant digits).

Variable names and default aesthetics

This stat illustrates another important point. If we want to make this stat usable with other geoms, we should return a variable called density instead of y. Then we can set up the default_aes to automatically map density to y, which allows the user to override it to use with different geoms:

However, using this stat with the area geom doesn’t work quite right. The areas don’t stack on top of each other:

This is because each density is computed independently, and the estimated xs don’t line up. We can resolve that issue by computing the range of the data once in setup_params().


  1. Extend stat_chull to compute the alpha hull, as from the alphahull package. Your new stat should take an alpha argument.

  2. Modify the final version of StatDensityCommon to allow the user to specify the min and max parameters. You’ll need to modify both the layer function and the compute_group() method.

    Note: be careful when adding parameters to a layer function. The following names col, color, pch, cex, lty, lwd, srt, adj, bg, fg, min, and max are intentionally renamed to accomodate base graphical parameter names. For example, a value passed as min to a layer appears as ymin in the setup_params list of params. It is recommended you avoid using these names for layer parameters.

  3. Compare and contrast StatLm to ggplot2::StatSmooth. What key differences make StatSmooth more complex than StatLm?

Creating a new geom

It’s harder to create a new geom than a new stat because you also need to know some grid. ggplot2 is built on top of grid, so you’ll need to know the basics of drawing with grid. If you’re serious about adding a new geom, I’d recommend buying R graphics by Paul Murrell. It tells you everything you need to know about drawing with grid.

A simple geom

It’s easiest to start with a simple example. The code below is a simplified version of geom_point():

This is very similar to defining a new stat. You always need to provide fields/methods for the four pieces shown above:

draw_panel() has three arguments:

You need to use panel_params and coord together to transform the data coords <- coord$transform(data, panel_params). This creates a data frame where position variables are scaled to the range 0–1. You then take this data and call a grid grob function. (Transforming for non-Cartesian coordinate systems is quite complex - you’re best off transforming your data to the form accepted by an existing ggplot2 geom and passing it.)

Collective geoms

Overriding draw_panel() is most appropriate if there is one graphic element per row. In other cases, you want graphic element per group. For example, take polygons: each row gives one vertex of a polygon. In this case, you should instead override draw_group().

The following code makes a simplified version of GeomPolygon:

There are a few things to note here:

You might want to compare this to the real GeomPolygon. You’ll see it overrides draw_panel() because it uses some tricks to make polygonGrob() produce multiple polygons in one call. This is considerably more complicated, but gives better performance.

Inheriting from an existing Geom

Sometimes you just want to make a small modification to an existing geom. In this case, rather than inheriting from Geom you can inherit from an existing subclass. For example, we might want to change the defaults for GeomPolygon to work better with StatChull:

This doesn’t allow you to use different geoms with the stat, but that seems appropriate here since the convex hull is primarily a polygonal feature.


  1. Compare and contrast GeomPoint with GeomSimplePoint.

  2. Compare and contrast GeomPolygon with GeomSimplePolygon.

Creating your own theme

If you’re going to create your own complete theme, there are a few things you need to know:

Overriding elements

By default, when you add a new theme element, it inherits values from the existing theme. For example, the following code sets the key colour to red, but it inherits the existing fill colour:

To override it completely, use %+replace% instead of +:

Global elements

There are four elements that affect the global appearance of the plot:

Element Theme function Description
line element_line() all line elements
rect element_rect() all rectangular elements
text element_text() all text
title element_text() all text in title elements (plot, axes & legend)

These set default properties that are inherited by more specific settings. These are most useful for setting an overall “background” colour and overall font settings (e.g. family and size).