ggplot2：仅将hline添加到一个网格

Question

假设以下数据：

library(ggplot2)

set.seed(1)
X <- data.frame(
    x     = 1:10,
    y     = sample(1:20, 10), 
    group = sample(paste0("g", 1:3), 10, replace = TRUE), 
    type  = rep(c("a", "b"), each = 5),
    style = rep(c(0, 1), each = 5)
)

> X
    x  y group type style
1   1  6    g1    a     0
2   2  8    g1    a     0
3   3 11    g3    a     0
4   4 16    g2    a     0
5   5  4    g3    a     0
6   6 14    g2    b     1
7   7 15    g3    b     1
8   8  9    g3    b     1
9   9 19    g2    b     1
10 10  1    g3    b     1

以下情节：

ggplot(X, aes(x, y)) +
    geom_point(aes(color = group)) +
    facet_grid(. ~ type)

看起来像这样：

目标是使用geom_hline添加yintercept = 10，但仅添加到左侧网格（其他所有内容应保持不变）。我尝试了下面的脚本：

ggplot(X, aes(x, y)) +
    geom_point(aes(color = group)) +
    geom_hline(aes(linetype = style), yintercept = 10) + # does not work
    # geom_hline(data = X[X$type == "b", ], yintercept = 10) + # doesn't work neither
    facet_grid(. ~ type)

但它为两个网格添加了一条水平线。任何解决方法？

解决方案：

geom_hline(data = data.frame(yint = 10, type = "b"), aes(yintercept = yint)) +