使用gganimate绘制含双geom_points的德国地图动态可视化时出现空白动画的问题求助

我最近尝试用gganimate制作一个包含两种geom_point的德国地图动态可视化，但最终只得到了空白的动画，实在搞不清哪里出问题了😭。

我想要实现的效果是：

• 绘制德国地图作为底图
• 用一组点根据variable_of_interest的值（大于0.3标红，否则标蓝）展示不同位置随时间的变化
• 另一组点用季节对应的emoji符号（比如雪花代表冬季）来显示当前季节

以下是我写的可复现代码，麻烦各位帮忙排查下问题：

# Load necessary packages
pacman::p_load(ggplot2, tidyverse, maps, mapdata,
               gganimate, av, emojifont)

# Production of example data
{
set.seed(123)
index = rep(c(1:10),
            each = 100) ; year = rep(c(1951:1960),
                                    each = 10, times = 10); month = rep(c(1:10),
                                                                       each = 1,
                                                                       times = 100)
variable_of_interest = runif(1000,
                             min = 0,
                             max = 0.4); season = rep(x = c("Winter", "Winter", "Spring", "Spring", "Spring",
                                                           "Summer", "Summer", "Summer", "Autmn", "Autmn"),
                                                     times = 100)
lat = rep(c(seq(from = 48,
                to = 53,
                length = 10)),
          each = 100); lon = rep(c(seq(from = 7,
                                       to = 14,
                                       length = 10)),
                                 each = 100)
Variable_of_Interest = as.data.frame(cbind(index, year, month, variable_of_interest, season, lat, lon))
Variable_of_Interest$month_of_year = paste0(Variable_of_Interest$year,
                                            "-",
                                            Variable_of_Interest$month)
rownames(Variable_of_Interest) = NULL
}

# Transform columns from character to numeric
Variable_of_Interest = mutate_at(Variable_of_Interest,
                                 c(1,2,3,4,6,7),
                                 as.numeric)

# Preperation for the animation
season_symbols = c("Spring" = emoji("cherry_blossom"),
                   "Summer" =  emoji("tropical_drink"),
                   "Autmn" = emoji("fallen_leaf"),
                   "Winter" = emoji("snowflake"))

# Map of values of the variable of interest
Variable_of_Interest_Map = ggplot(Variable_of_Interest) +
  geom_path(data = map_data("world","Germany"),
            aes(x = long,
                y = lat,
                group = group)) +
  coord_fixed(xlim = c(6,
                       15),
              ylim = c(47,
                       55)) +
  geom_point(aes(x=lon,
                 y=lat,
                 group = month_of_year,
                 shape = season),
             size = 10) +
  scale_shape_manual(values = season_symbols, breaks = c("Spring", "Summer", "Autmn", "Winter")) +
  geom_point(aes(x=lon,
                 y=lat,
                 group = month_of_year),
             colour = ifelse(test = Variable_of_Interest$variable_of_interest > 0.3,
                            I("red"),
                            I("blue"))) +
  # scale_color_gradient(low="blue", high="yellow") +
  xlab("Longitude (degree)") +
  ylab("Latitude (degree)") +
  theme_bw() +
  transition_manual(frames = month_of_year) +
  theme(legend.position = "bottom",
        legend.title = element_text(size=12, face="bold"),
        panel.background = element_blank(),
        legend.background = element_blank()) +
  labs(title = '{unique(Variable_of_Interest$month_of_year)[as.integer(frame)]}',
       color = paste0("Variable of Interest"),
       shape = "season")

anim_save("Variable-of-Interest.mp4",
          animate(
            Variable_of_Interest_Map,
            nframes = 1000,
            renderer = ffmpeg_renderer()
          ))

现在运行这段代码后生成的动画完全是空白的，没有任何地图或点显示，有没有大佬能帮我找出问题所在呀？

备注：内容来源于stack exchange，提问作者RibbitRibbit