Follow-up to #353, create a geofacet with Guerry data

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I'm re-opening this query #353 after I have what I think is a solution, but I get an error when I try to use it.
The code below creates gfrance85_grid from the map data Guerry::gfrance85.

#' ---
#' title: Create gfrance grid for geofacet package
#' ---
#' 

library(geofacet)
library(ggplot2)
library(dplyr)
library(tidyr)
library(Guerry)


# scale a variable into n integer bins
bin <- function(x, n=10){
  1 + floor(n * (x - min(x)) / (max(x) - min(x)))
}

data(gfrance85, package = "Guerry")

dept         <- data.frame(gfrance85)[,"dept"]   
dep.names    <- data.frame(gfrance85)[,"Department"]
region.names <- data.frame(gfrance85)[,"Region"]


# extract department centroids
xy <- coordinates(gfrance85) |>
  as.data.frame() |>
  setNames(c("x", "y")) 

gfrance85_grid <- data.frame(code = dept,
                             name = as.character(dep.names),
                             row = bin(xy$x),
                             col = bin(xy$y))
head(gfrance85_grid)

grid_preview("gfrance85_grid") +
  geom_text(aes(label=name), size=2, nudge_y=0.35)

This looks OK to me:

> head(gfrance85_grid)
  code         name row col
1    1          Ain   8   4
2    2        Aisne   6   8
3    3       Allier   6   4
4    4 Basses-Alpes   9   1
5    5 Hautes-Alpes   9   2
6    7      Ardeche   7   2

grid_preview gives me this error, suggesting it does not accept an appropriate grid data frame as input, but instead looks inside the package:

> grid_preview("gfrance85_grid") +
+   geom_text(aes(label=name), size=2, nudge_y=0.35)
grid 'gfrance85_grid' not found in package, checking online...
Error in get_grid(x) : grid 'gfrance85_grid' not recognized...