See https://rud.is/b/2017/09/18/mapping-fall-foliage-with-sf/

mapping-with-magick.R

library(rprojroot)
library(sf)
library(magick)
library(tidyverse) # NOTE: Needs github version of ggplot2

root <- find_rstudio_root_file()
dir.create("data", showWarnings = FALSE)

# "borrow" the files from SmokyMountains.com, but be nice and cache them to
# avoid hitting their web server for every iteration

c("https://smokymountains.com/wp-content/themes/smcom-2015/to-delete/js/us.json",
  "https://smokymountains.com/wp-content/themes/smcom-2015/js/foliage2.tsv",
  "https://smokymountains.com/wp-content/themes/smcom-2015/js/foliage-2017.csv") %>%
  walk(~{
    sav_tmp <- file.path(root, "data", basename(.x))
    if (!file.exists(sav_tmp)) download.file(.x, sav_tmp)
  })

# next, we read in the GeoJSON file twice. first, to get the counties
states_sf <- read_sf(file.path(root, "data", "us.json"), "states", stringsAsFactors = FALSE)

# we only want the continental US
states_sf <- filter(states_sf, !(id %in% c("2", "15", "72", "78")))

# it doesn't have a CRS so we give it one
st_crs(states_sf) <- 4326

# I ran into hiccups using coord_sf() to do this, so we convert it to Albers here
states_sf <- st_transform(states_sf, 5070)


# next we read in the states
counties_sf <- read_sf(file.path(root, "data", "us.json"), "counties", stringsAsFactors = FALSE)
st_crs(counties_sf) <- 4326
counties_sf <- st_transform(counties_sf, 5070)

# now, we read in the foliage data
foliage <- read_tsv(file.path(root, "data", "foliage-2017.csv"),
                    col_types = cols(.default=col_double(), id=col_character()))

# and, since we have a lovely sf tidy data frame, bind it together
left_join(counties_sf, foliage, "id") %>%
  filter(!is.na(rate1)) -> foliage_sf

# now, we do some munging so we have better labels and so we can
# iterate over the weeks
gather(foliage_sf, week, value, -id, -geometry) %>%
  mutate(value = factor(value)) %>%
  filter(week != "rate1") %>%
  mutate(week = factor(week,
                       levels=unique(week),
                       labels=format(seq(as.Date("2017-08-26"),
                                         as.Date("2017-11-11"), "1 week"),
                                     "%b %d"))) -> foliage_sf

# now we start the graphics device
frames <- image_graph(width = 1500, height = 900, res = 300)

# make a ggplot object for each week and print the graphic
pb <- progress_estimated(nlevels(foliage_sf$week))
walk(1:nlevels(foliage_sf$week), ~{
  
  pb$tick()$print()
  
  xdf <- filter(foliage_sf, week == levels(week)[.x])
  
  ggplot() +
    geom_sf(data=xdf, aes(fill=value), size=0.05, color="#2b2b2b") +
    geom_sf(data=states_sf, color="white", size=0.125, fill=NA) +
    viridis::scale_fill_viridis(
      name=NULL,
      discrete = TRUE,
      labels=c("No Change", "Minimal", "Patchy", "Partial", "Near Peak", "Peak", "Past Peak"),
      drop=FALSE
    ) +
    labs(title=sprintf("Foliage: %s ", unique(xdf$week))) +
    ggthemes::theme_map() +
    theme(panel.grid=element_line(color="#00000000")) +
    theme(panel.grid.major=element_line(color="#00000000")) +
    theme(legend.position="right") -> gg
  
  print(gg)
  
})

# animate the foliage
image_animate(frames, 1)