2.8 Output

Most of the time you create a plot object and immediately plot it, but you can also save a plot to a variable and manipulate it:

p <- ggplot(mpg, aes(displ, hwy, colour = factor(cyl))) +
  geom_point()

Once you have a plot object, there are a few things you can do with it:

• Render it on screen with print(). This happens automatically when running interactively, but inside a loop or function, you’ll need to print() it yourself.

  print(p)

  

• Save it to disk with ggsave(), described in Section 17.5.

  # Save png to disk
  ggsave("plot.png", p, width = 5, height = 5)

• Briefly describe its structure with summary().

  summary(p)
  #> data: manufacturer, model, displ, year, cyl, trans, drv, cty, hwy, fl,
  #>   class [234x11]
  #> mapping:  x = ~displ, y = ~hwy, colour = ~factor(cyl)
  #> faceting: <ggproto object: Class FacetNull, Facet, gg>
  #>     compute_layout: function
  #>     draw_back: function
  #>     draw_front: function
  #>     draw_labels: function
  #>     draw_panels: function
  #>     finish_data: function
  #>     init_scales: function
  #>     map_data: function
  #>     params: list
  #>     setup_data: function
  #>     setup_params: function
  #>     shrink: TRUE
  #>     train_scales: function
  #>     vars: function
  #>     super:  <ggproto object: Class FacetNull, Facet, gg>
  #> -----------------------------------
  #> geom_point: na.rm = FALSE
  #> stat_identity: na.rm = FALSE
  #> position_identity

• Save a cached copy of it to disk, with saveRDS(). This saves a complete copy of the plot object, so you can easily re-create it with readRDS().

  saveRDS(p, "plot.rds")
  q <- readRDS("plot.rds")

You’ll learn more about how to manipulate these objects in Chapter 18.