R/autoplot.R

In ggplot2: Create Elegant Data Visualisations Using the Grammar of Graphics

#' Tailoring plots to particular data types
#'
#' @description
#' There are three functions to make plotting particular data types easier:
#' `autoplot()`, `autolayer()` and `fortify()`. These are S3 generics for which
#' other packages can write methods to display classes of data. The three
#' functions are complementary and allow different levels of customisation.
#' Below we'll explore implementing this series of methods to automate plotting
#' of some class.
#'
#' Let's suppose we are writing a packages that has a class called 'my_heatmap',
#' that wraps a matrix and we'd like users to easily plot this heatmap.
#'
#' ```r
#' my_heatmap <- function(...) {
#'   m <- matrix(...)
#'   class(m) <- c("my_heatmap", class(m))
#'   m
#' }
#'
#' my_data <- my_heatmap(volcano)
#' ```
#'
#' # Automatic data shaping
#'
#' One of the things we have to do is ensure that the data is shaped in the long
#' format so that it is compatible with ggplot2. This is the job of the
#' `fortify()` function. Because 'my_heatmap' wraps a matrix, we can let the
#' fortify method 'melt' the matrix to a long format. If your data is already
#' based on a long-format `<data.frame>`, you can skip implementing a
#' `fortify()` method.
#'
#' ```r
#' fortify.my_heatmap <- function(model, ...) {
#'   data.frame(
#'     row = as.vector(row(model)),
#'     col = as.vector(col(model)),
#'     value = as.vector(model)
#'   )
#' }
#'
#' fortify(my_data)
#' ```
#'
#' When you have implemented the `fortify()` method, it should be easier to
#' construct a plot with the data than with the matrix.
#'
#' ```r
#' ggplot(my_data, aes(x = col, y = row, fill = value)) +
#'   geom_raster()
#' ```
#'
#' # Automatic layers
#'
#' A next step in automating plotting of your data type is to write an
#' `autolayer()` method. These are typically wrappers around geoms or stats
#' that automatically set aesthetics or other parameters. If you haven't
#' implemented a `fortify()` method for your data type, you might have to
#' reshape the data in `autolayer()`.
#'
#' If you require multiple layers to display your data type, you can use an
#' `autolayer()` method that constructs a list of layers, which can be added
#' to a plot.
#'
#' ```r
#' autolayer.my_heatmap <- function(object, ...) {
#'   geom_raster(
#'     mapping = aes(x = col, y = row, fill = value),
#'     data = object,
#'     ...,
#'     inherit.aes = FALSE
#'   )
#' }
#'
#' ggplot() + autolayer(my_data)
#' ```
#'
#' As a quick tip: if you define a mapping in `autolayer()`, you might want
#' to set `inherit.aes = FALSE` to not have aesthetics set in other layers
#' interfere with your layer.
#'
#' # Automatic plots
#'
#' The last step in automating plotting is to write an `autoplot()` method
#' for your data type. The expectation is that these return a complete plot.
#' In the example below, we're exploiting the `autolayer()` method that we
#' have already written to make a complete plot.
#'
#' ```r
#' autoplot.my_heatmap <- function(object, ..., option = "magma") {
#'   ggplot() +
#'     autolayer(my_data) +
#'     scale_fill_viridis_c(option = option) +
#'     theme_void()
#' }
#'
#' autoplot(my_data)
#' ```
#'
#' If you don't have a wish to implement a base R plotting method, you
#' can set the plot method for your class to the autoplot method.
#'
#' ```r
#' plot.my_heatmap <- autoplot.my_heatmap
#' plot(my_data)
#' ```
#'
#' @family plotting automation topics
#'
#' @name automatic_plotting
NULL

#' Create a complete ggplot appropriate to a particular data type
#'
#' `autoplot()` uses ggplot2 to draw a particular plot for an object of a
#' particular class in a single command. This defines the S3 generic that
#' other classes and packages can extend.
#'
#' @param object an object, whose class will determine the behaviour of autoplot
#' @param ... other arguments passed to specific methods
#' @return a ggplot object
#' @export
#' @family plotting automation topics
autoplot <- function(object, ...) {
  UseMethod("autoplot")
}

#' @export
autoplot.default <- function(object, ...) {
  cli::cli_abort(c(
    "Objects of class {.cls {class(object)[[1]]}} are not supported by autoplot.",
    "i" = "Have you loaded the required package?"
  ))
}