将ggplot图例的一部分加粗

我想制作一部分图例标签(存储在单独的 data.frame ). 这 question 是相关的,但它比我的情况更简单:

library(tidyverse)
df <- data.frame(xx = c(0, 3),
                 yy = c(0, 1))

df_text <- data.frame(start = c(0, 1, 2),
                      end = c(1, 2, 3),
                      heading_bold = c("title 1", "title 2", "title 3"),
                      normal_text = c("i want this to be normal text title 1",
                                      "i want this to be normal text title 2",
                                      "i want this to be normal text title 3")) %>% 
  mutate(merged_text = paste0(heading_bold, ": ", normal_text)) #%>% 
  # perhaps I should try and make bold here?
  #mutate(merged_text = paste0("bold(", heading_bold, ")", ": ", normal_text))

ggplot(df, aes(x = xx, y = yy)) +
  geom_rect(data = df_text,
            aes(xmin = start,
                xmax = end,
                fill = merged_text,
                ymin = 0,
                ymax = 1),
            inherit.aes = FALSE) +
  scale_fill_viridis_d(option = "A", name = "Legend", limits = unique(df_text$merged_text), direction = -1) 

我想要 title 1 , title 2 和 title 3 在传说中大胆一点。

正如我在上面注释掉的代码中所建议的那样,一种可能的方法可能是将文本包装在 bold 作用

mutate(merged_text = paste0("bold(", heading_bold, ")", ": ", normal_text))

但我不知道该怎么称呼这个表达。

有什么想法吗?

谢谢