Examples gallery

library(ggtrace)
library(ggplot2)
packageVersion("ggplot2")
#> [1] '4.0.3'

Highjack layer data

Example inspired by a stackoverflow question:

bars <- ggplot(mpg, aes(class)) +
  geom_bar(aes(fill = drv), color = "grey")
bars

Intercepting the data at draw step to subset bars arbitrarily:

bars_subset <- highjack_args(
  x = bars, method = Geom$draw_layer, cond = 1L,
  values = expression(
    data = data[c(2, 4, 6, 8, 11),]
  )
)
bars_subset

Debug complex `aes()` expressions

Example adopted from my rstudio::conf 2022 talk

Given a boxplot made with a geom_boxplot() layer, suppose that we want to add a second layer annotating the value of the upper whiskers:

box_p <- ggplot(data = mtcars) +
  aes(x = factor(cyl), y = mpg) +
  geom_boxplot()
box_p

A naive approach would be to add a layer that combines a boxplot stat with a label geom. But this errors out of the box:

box_p +
  geom_label(stat = "boxplot")
#> Error in `geom_label()`:
#> ! Problem while setting up geom.
#> ℹ Error occurred in the 2nd layer.
#> Caused by error in `compute_geom_1()`:
#> ! `geom_label()` requires the following missing aesthetics: y and label.

The error tells us that the geom is missing some missing aesthetics, so something must be wrong with the data that the geom receives. If we inspect this using layer_before_geom(), we find that the columns for y and label are indeed missing in the Before Geom data:

layer_before_geom(last_plot(), i = 2L, error = TRUE, verbose = TRUE)
#> ✔ Ran `inspect_args(last_plot(), ggplot2:::Layer$compute_geom_1, layer_is(2L), error = TRUE)$data`
#> # A tibble: 3 × 14
#>    ymin lower middle upper  ymax outliers  notchupper notchlower     x width
#>   <dbl> <dbl>  <dbl> <dbl> <dbl> <list>         <dbl>      <dbl> <dbl> <dbl>
#> 1  21.4  22.8   26    30.4  33.9 <dbl [0]>       29.6       22.4     1  0.75
#> 2  17.8  18.6   19.7  21    21.4 <dbl [0]>       21.1       18.3     2  0.75
#> 3  13.3  14.4   15.2  16.2  18.7 <dbl [3]>       16.0       14.4     3  0.75
#> # ℹ 4 more variables: relvarwidth <dbl>, flipped_aes <lgl>, PANEL <fct>,
#> #   group <int>

Note that you can more conveniently call last_layer_errorcontext() to the same effect:

last_layer_errorcontext()
#> ✔ Ran `inspect_args(last_plot(), ggplot2:::Layer$compute_geom_1, layer_is(2L), error = TRUE)$data`
#> # A tibble: 3 × 14
#>    ymin lower middle upper  ymax outliers  notchupper notchlower     x width
#>   <dbl> <dbl>  <dbl> <dbl> <dbl> <list>         <dbl>      <dbl> <dbl> <dbl>
#> 1  21.4  22.8   26    30.4  33.9 <dbl [0]>       29.6       22.4     1  0.75
#> 2  17.8  18.6   19.7  21    21.4 <dbl [0]>       21.1       18.3     2  0.75
#> 3  13.3  14.4   15.2  16.2  18.7 <dbl [3]>       16.0       14.4     3  0.75
#> # ℹ 4 more variables: relvarwidth <dbl>, flipped_aes <lgl>, PANEL <fct>,
#> #   group <int>

Thus, we need to ensure that y exists to satisfy both the stat and the geom, and that label exists after the statistical transformation step but before the geom sees the data. Crucially, we use the computed variable ymax to (re-)map to the y and label aesthetics.

box_p +
  geom_label(
    aes(y = stage(mpg, after_stat = ymax),
        label = after_stat(ymax)),
    stat = "boxplot"
  )

Inspecting the after-stat snapshot of the successful plot above, we see that both y and label are now present at this stage to later satisfy the geom.

layer_before_geom(last_plot(), i = 2L)[, c("y", "ymax", "label")]
#> # A tibble: 3 × 3
#>       y  ymax label
#>   <dbl> <dbl> <dbl>
#> 1  33.9  33.9  33.9
#> 2  21.4  21.4  21.4
#> 3  18.7  18.7  18.7

Inspect sub-layer data

Example adopted from Demystifying delayed aesthetic evaluation

A bar plot of counts with geom_bar() with stat = "count" default:

bar_plot <- ggplot(mpg, aes(class, fill = class)) +
  geom_bar() +
  theme(legend.position = "none")

State of bar layer’s data after the statistical transformation step:

layer_after_stat(bar_plot, verbose = TRUE)
#> ✔ Ran `inspect_return(bar_plot, ggplot2:::Layer$compute_statistic, layer_is(1L))`
#> # A tibble: 7 × 8
#>   count  prop x          width flipped_aes fill       PANEL group
#>   <dbl> <dbl> <mppd_dsc> <dbl> <lgl>       <chr>      <fct> <int>
#> 1     5     1 1            0.9 FALSE       2seater    1         1
#> 2    47     1 2            0.9 FALSE       compact    1         2
#> 3    41     1 3            0.9 FALSE       midsize    1         3
#> 4    11     1 4            0.9 FALSE       minivan    1         4
#> 5    33     1 5            0.9 FALSE       pickup     1         5
#> 6    35     1 6            0.9 FALSE       subcompact 1         6
#> 7    62     1 7            0.9 FALSE       suv        1         7

We can map aesthetics to variables from the after-stat data using after_stat():

bar_plot +
  geom_text(
    aes(label = after_stat(count)),
    stat = "count",
    position = position_nudge(y = 1), vjust = 0
  )

Intervene with surgical precision

Example adopted from a twitter thread

Here’s a plot in polar coordinates:

polar_plot <- ggplot(mtcars, aes(hp, mpg)) +
  geom_point() +
  geom_smooth(method = "lm", formula = y ~ x) +
  expand_limits(y = c(0, 60)) +
  coord_polar(start = 0, theta = "y")

polar_plot

We can clip the plot panel by highjacking the Layout$render() method using the generic workflow function with_ggtrace():

with_ggtrace(
  x = polar_plot + theme(aspect.ratio = 1/.48),
  method = Layout$render,
  trace_steps = 5L,
  trace_expr = quote({
    panels[[1]] <- editGrob(panels[[1]], vp = viewport(xscale = c(.48, 1)))
  }),
  out = "g"
)

See implementation in MSBMisc::crop_coord_polar().

Highjack the drawing context

Example adopted from my useR! 2022 talk:

library(palmerpenguins)
#> 
#> Attaching package: 'palmerpenguins'
#> The following objects are masked from 'package:datasets':
#> 
#>     penguins, penguins_raw
flashy_plot <- na.omit(palmerpenguins::penguins) |> 
  ggplot(aes(x = species, y = flipper_length_mm)) +
  geom_boxplot(aes(fill = species), width = .7) +
  facet_wrap(~ year)
flashy_plot

highjack_return(
  flashy_plot, Geom$draw_panel, cond = TRUE,
  value = quote({
    circ <- circleGrob(y = .25 * ._counter_)
    grobTree( editGrob(circ, gp = gpar(fill = linearGradient())),
              editGrob(returnValue(), vp = viewport(clip = circ)) )
  }))

Note the use of the special variable ._counter_, which increments every time a function/method has been called. See the tracing context topic for more details.