r - geom_line 渐变变化点

Question

取以下数据

df <- data.frame(ID = c(1, 1, 2, 2, 3, 3),
                 cond = c(2, 2, 2, 2, 1, 1),
                 Time = c(1,2,1,2,1,2),
                 State  = c("Empty", "Empty", "Empty", "Full", "Full", "Empty"),
                 eye = c(2, -3, -2, 1, 0, -2),
                 combination = c("1", "1","2", "2", "3", "3"))
df$cond <- factor(df$cond,levels = c("1", "2"))

ggplot(df, aes(x = Time, y = eye)) +
  ggforce::geom_link2(aes(group=ID, color = cond, alpha = State), size = 5, n = 500, lineend = "round") +
  scale_color_manual(values=c("#CC6666", "#9999CC")) +
  scale_alpha_discrete(range = c(0.02, 0.7)) +
  geom_point(aes(shape=State), size = 3.5) +
  scale_shape_manual(values=c(13,15)) +
  
    theme(legend.title=element_blank(),
        axis.title.x=element_text(size = 12),
        axis.title.y=element_text(size = 12),
        plot.title = element_text(hjust = 0.5, size = 15),
        axis.text.x= element_text(color = "black"),
        axis.text.y= element_text(color = "black"),
        axis.line = element_line(colour = "black"),
        plot.background = element_rect(fill = "white"),
        panel.background = element_rect(fill = "white"))

这将产生以下图片：

问题是对于第 2 行和第 3 行，alpha 达到 1 太快（几乎没有部分行是透明的）。我正在寻找一种方法来更改 alpha 如何跨行变化，以便对于第 2 行和第 3 行，更多的行保持在“低 alpha” - 最好，该行应该只在 1.5 点开始变化（x 轴）。

我在这里不知所措，非常感谢任何帮助。