expect_doppelganger() is a testthat expectation for graphical plots. It generates SVG snapshots that you can review graphically with testthat::snapshot_review(). You will find more information about snapshotting in the testthat snapshots vignette.

## Usage

expect_doppelganger(
title,
fig,
path = deprecated(),
...,
writer = write_svg,
cran = FALSE
)

## Arguments

title

A brief description of what is being tested in the figure. For instance: "Points and lines overlap".

If a ggplot2 figure doesn't have a title already, title is applied to the figure with ggtitle().

The title is also used as file name for storing SVG (in a sanitzed form, with special characters converted to "-").

fig

A figure to test. This can be a ggplot object, a recordedplot, or more generally any object with a print method.

If you need to test a plot with non-printable objects (e.g. base plots), fig can be a function that generates and prints the plot, e.g. fig = function() plot(1:3).

path, ...

.

writer

A function that takes the plot, a target SVG file, and an optional plot title. It should transform the plot to SVG in a deterministic way and write it to the target file. See write_svg() (the default) for an example.

cran

If FALSE (the default), mismatched snapshots only cause a failure when you run tests locally or in your CI (Github Actions or any platform that sets the CI environment variable). If TRUE, failures may also occur on CRAN machines.

Failures are disabled on CRAN by default because testing the appearance of a figure is inherently fragile. Changes in the R graphics engine or in ggplot2 may cause subtle differences in the aspect of a plot, such as a slightly smaller or larger margin. These changes will cause spurious failures because you need to update your snapshots to reflect the upstream changes.

It would be distracting for both you and the CRAN maintainers if such changes systematically caused failures on CRAN. This is why snapshot expectations do not fail on CRAN by default and should be treated as a monitoring tool that allows you to quickly check how the appearance of your figures changes over time, and to manually assess whether changes reflect actual problems in your package.

Internally, this argument is passed to testthat::expect_snapshot_file().

## Debugging

It is sometimes difficult to understand the cause of a failure. This usually indicates that the plot is not created deterministically. Potential culprits are:

• Some of the plot components depend on random variation. Try setting a seed.

• The plot depends on some system library. For instance sf plots depend on libraries like GEOS and GDAL. It might not be possible to test these plots with vdiffr.

To help you understand the causes of a failure, vdiffr automatically logs the SVG diff of all failures when run under R CMD check. The log is located in tests/vdiffr.Rout.fail and should be displayed on Travis.

You can also set the VDIFFR_LOG_PATH environment variable with Sys.setenv() to unconditionally (also interactively) log failures in the file pointed by the variable.

## Examples

if (FALSE) {  # Not run

library("ggplot2")

test_that("plots have known output", {
disp_hist_base <- function() hist(mtcars\$disp)
expect_doppelganger("disp-histogram-base", disp_hist_base)

disp_hist_ggplot <- ggplot(mtcars, aes(disp)) + geom_histogram()
expect_doppelganger("disp-histogram-ggplot", disp_hist_ggplot)
})

}