向 R 绘图热图中添加水平线

我正在try 将水平线添加到R plotly heatmap，它将位于heatmap的几行之间.

下面是示例data.frame和heatmap:

library(plotly)
set.seed(1)
df <- matrix(rnorm(18), nrow = 6, ncol = 3, dimnames = list(paste0("r",1:6),1:3)) %>%
  reshape2::melt() %>%
  dplyr::rename(row=Var1,col=Var2)
plot_ly(x = df$col, y = df$row,z = df$value,type = "heatmap")

Which gives:

现在假设我想在"r2"和"r3"之间添加一条横穿整个heatmap的水平线，在"r4"和"r5"之间添加一条类似的线.

我不知道对应的y个位置应该是什么.

如果我的df$row岁是integer/numeric岁而不是character岁，我就能做到这一点:

library(plotly)
set.seed(1)
df <- matrix(rnorm(18), nrow = 6, ncol = 3, dimnames = list(1:6,1:3)) %>%
  reshape2::melt() %>%
  dplyr::rename(row=Var1,col=Var2)
plot_ly(x = df$col, y = df$row,z = df$value,type = "heatmap") %>%
  add_lines(y = 2.5, x = c(min(df$col)-0.5,max(df$col)+0.5), line = list(color = "black",dash = "dot",size = 5),inherit = FALSE,showlegend = FALSE) %>%
  add_lines(y = 4.5, x = c(min(df$col)-0.5,max(df$col)+0.5), line = list(color = "black",dash = "dot",size = 5),inherit = FALSE,showlegend = FALSE)

所以我的问题是:

1. 如果热图的行数为character，有没有办法在行之间放置水平线？
2. 有没有一种更紧凑的方法来添加多条水平线，而不是像我上面的代码那样显式地编码每一条？