Allow for easy insertion of ggplot plots in a table column via a formatter

[rich-iannone]

The new fmt_ggplot() function allows for easier inclusion of ggplot2 plots (within a list column of the input table data) into a gt table. The common pattern toward obtaining these plots is through
mutation of a list column containing all the data required for plot:

<data> %>% 
  dplyr::group_by(<var>) %>%
  tidyr::nest(.key = plot) %>%
  dplyr::mutate(plot = purrr::map(plot, <ggplot code>))

While the PR contains functionality to detect and automatically format columns containing ggplot2 plots, we can also opt to use the fmt_ggplot() function to allow us to specify rows and set options for the plots' height and aspect_ratio.

Here are some examples of how this would work:

# Create a small table with a `plot` column
gtcars_plot_column <-
  gtcars %>%
  dplyr::group_by(mfr) %>%
  tidyr::nest(.key = plot) %>%
  dplyr::mutate(
    plot = map(plot, ~ggplot(., aes(hp, trq)) + geom_point())) %>%
  head(3)

# Automatic formatting of plots in `plot` using
# default options
tab_1 <- 
  gtcars_plot_column %>%
  gt()

# Using `fmt_ggplot()` to provide values
# for `columns`, `height` and `aspect_ratio`
tab_2 <- 
  gtcars_plot_column %>%
  gt() %>%
  fmt_ggplot(
    columns = vars(plot),
    height = 200,
    aspect_ratio = 2.5
  )

Here are images of tab_1 and tab_2:

These are minimal examples showing the basic usage of inserting plots, so, the resulting tables look less than ideal. With some extra care given to setting common axis ranges and using ggplot2::theme() to increase font sizes, line widths, etc., inserted plots would become more aesthetically pleasing.

Fixes #152

[jcheng5]

As discussed in person, I think all such features shouldn't be specific to ggplot but should work with all static R plots.

[GregSutcliffe]

@rich-iannone any update on this? embedded plots came up again for me today, and I'm getting by with a rebased version of this branch... can I help at all?

[ciallen]

Hey! Any updates to this? I need to embed box plots in a table and would love to use this function. Thanks!

[ymer]

It would be nice if this could be pushed. I think 90%+ of the use would be for ggplot, and it works well for that.