热图（geom_tile）产生黑条覆盖图

Question

我正在尝试使用ggplot生成热图，以显示随时间变化的值。我的大部分情节都被大黑条覆盖。如何删除这些栏以仅看到红色和绿色？

set.seed(1)

df <- data.frame(datetime = seq(dmy_hm("12/04/2020 12:00"), dmy_hm("12/04/2020 13:00"), "5 min"),
                   bund = as.factor("P1"),
                   inflow = (rnorm(13)))

plot <- ggplot(df, aes(x = datetime, y = bund,  colour = inflow)) + 
  geom_tile(height = 0.9) +
  scale_colour_gradient2(low = "red", high = "green", midpoint = 0)

plot