问题原因是ggplot2不支持在单个图层中填充透明图形。解决方法是将矩形拆分成多个图层，并使用透明度来获得填充效果。

下面是代码示例：

library(ggplot2) library(gganimate)

df <- data.frame(x = c(1,3,2,4), y = c(1,2,3,4), grp = c("A", "A", "B", "B"))

Create list of individual plots with each polygon

plot_list <- lapply(unique(df$grp), function(grp){ ggplot(df[df$grp == grp,]) + geom_rect(aes(xmin = x - 0.5, xmax = x + 0.5, ymin = y - 0.5, ymax = y + 0.5, fill = grp), alpha = 0.5) + ggtitle(grp) })

Stack plots on top of each other and apply animation

gg_animate(do.call(c, plot_list), nframes = length(plot_list), interval = 0.1)