Make sure displaying ggplot objects in a data frame doesn't error

[teunbrand]

per davidgohel/flextable#686 (comment):

library(ggplot2)
z <- iris
z$gg <- list(ggplot())
print(z)
#> Error in format.default(unlist(xx), ...): c("Found no format() method for class \"ggplot2::ggplot\"", "Found no format() method for class \"ggplot\"", "Found no format() method for class \"ggplot2::gg\"", "Found no format() method for class \"S7_object\"", "Found no format() method for class \"gg\"")

^{Created on 2025-06-20 with reprex v2.1.1}

[teunbrand]

It is actually an S7 problem:

library(S7)

foo <- new_class("foo")

df <- vctrs::data_frame(
  x = c(1, 2),
  y = list(foo(), foo())
)

print(df)
#> Error in format.default(unlist(xx), ...): c("Found no format() method for class \"foo\"", "Found no format() method for class \"S7_object\"")

^{Created on 2025-06-20 with reprex v2.1.1}

But I think we should fix it in ggplot2 until S7 patches this.
Cross-posted this in RConsortium/S7#548.

Now to figure out how one would fix this, as it appears we're forced to format.default() which doesn't have a switch for S7 objects.