library(sp)
library(rgdal)
library(mapview)
# GeoJSON file with required extent
file = "https://gist.githubusercontent.com/anonymous/c7b97b330f9bff08b595afbbdccb79a2/raw/1e86ea6aad1f802b3801fd5502654d0eece5eb30/map.geojson"
# Read file
x = readOGR(file, "OGRGeoJSON", verbose = FALSE)
# Create gridlines with ~29 breaks on the y-axis
grid = gridlines(x, norths = pretty(bbox(x)[1,], n = 29))
# View result on map
mapview(grid[1, ])