如何在ggplot条形图中调整各条形块的标签至居中位置？

我有如下数据集，想要绘制以AA为y轴、FCXM_no为x轴的条形图，要求每个条形块内的Specificity_uniqueaa标签居中对齐。但我编写的ggplot代码无法实现标签居中，请求调整代码。

数据集dput代码：

dput(FCXM_AA_sp)
structure(list(Specificity_uniqueaa = structure(1:54, levels = c("A3_D161", 
"A*30:01_W152", "B57_V97", "Cw5_K138", "Cw9_R91", "B13_L145", 
"A*11:02_K19", "B18_G30", "A2_H74", "B63_F4", "B44_V199", "Cw16_L193", 
"DR8_L74", "DQ4_D71", "A80_S31", "A80_E56", "A80_N74", "A80_K253", 
"A80_K255", "Cw1_K6", "Cw1_C99", "Cw1_M248", "Cw7_L194", "Cw7_M261", 
"Cw7_S273", "DR7_K14", "DR7_Q25", "DR7_L30", "DR10_E10", "DR10_R30", 
"DR10_V31", "DQ2_S28", "DQ2_S30", "DQ2_I37", "DQ2_L52", "DQB1*06:01_P3", 
"DQB1*06:01_L9", "DR4_H13", "DR4_H33", "DR4_Y96", "DR51_Q9", 
"DR51_Q191", "DR53_A11", "DR53_C13", "DR53_L18", "DR53_W25", 
"DR53_N26", "DR53_I28", "DR53_N41", "DR53_Q48", "DR53_Y81", "DR53_Q187", 
"DQ5_L14", "DQ5_S125"), class = "factor"), AA = structure(c(17L, 
2L, 5L, 14L, 15L, 4L, 14L, 10L, 13L, 1L, 5L, 4L, 4L, 17L, 12L, 
16L, 19L, 14L, 14L, 14L, 8L, 7L, 4L, 7L, 12L, 14L, 18L, 4L, 16L, 
15L, 5L, 12L, 12L, 3L, 4L, 9L, 4L, 13L, 13L, 11L, 18L, 18L, 6L, 
8L, 4L, 2L, 19L, 3L, 19L, 18L, 11L, 18L, 4L, 12L), levels = c("F", 
"W", "I", "L", "V", "A", "M", "C", "P", "G", "Y", "S", "H", "K", 
"R", "E", "D", "Q", "N"), class = "factor"), FCXM = c("Negative", 
"Negative", "Positive", "Negative", "Negative", "Negative", "Negative", 
"Positive", "Positive", "Negative", "Negative", "Negative", "Negative", 
"Negative", "Negative", "Negative", "Negative", "Negative", "Negative", 
"Positive", "Positive", "Positive", "Negative", "Negative", "Negative", 
"Positive", "Positive", "Positive", "Positive", "Positive", "Positive", 
"Positive", "Positive", "Positive", "Positive", "Negative", "Negative", 
"Positive", "Positive", "Positive", "Negative", "Negative", "Negative", 
"Negative", "Negative", "Negative", "Negative", "Negative", "Negative", 
"Negative", "Negative", "Negative", "Negative", "Negative"), 
    FCXM_no = c(-1L, -1L, 1L, -1L, -1L, -1L, -1L, 1L, 1L, -1L, 
    -1L, -1L, -1L, -1L, -1L, -1L, -1L, -1L, -1L, 1L, 1L, 1L, 
    -1L, -1L, -1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, -1L, 
    -1L, 1L, 1L, 1L, -1L, -1L, -1L, -1L, -1L, -1L, -1L, -1L, 
    -1L, -1L, -1L, -1L, -1L, -1L)), row.names = c(NA, -54L), class = "data.frame")

期望效果：每个AA对应的条形块（按FCXM正负区分）内，Specificity_uniqueaa标签居中显示，条形图以AA为y轴、FCXM_no为x轴。

现有ggplot代码：

ggplot(FCXM_AA_sp, aes(y = FCXM_no, x = AA, fill = FCXM)) +   # Fill column
  geom_bar(stat = "identity", width = .6, color= "white", size=0.5) +   # draw the bars
  geom_text(aes(label=paste0(Specificity_uniqueaa,"")),nudge_y=10,colour = "black", size = 4, hjust = 8.4, vjust = 1) + #angle=90
  scale_y_discrete() + # Labels
  scale_x_discrete() + # Labels
  coord_flip() +  # Flip axes
  labs(title="FCXM vs AA") +
  theme_tufte() +  # Tufte theme from ggfortify
  theme(plot.title = element_text(hjust = .5), 
        axis.ticks = element_blank()) +   # Centre plot title
  scale_fill_brewer(palette = "Dark2")  # Color palette

原代码的问题在于使用了固定的nudge_y和不合理的hjust值，导致标签无法自动适配条形块位置。我们可以通过position_stack(vjust = 0.5)让文本自动居中于每个条形块内，同时调整轴映射的顺序（避免混淆coord_flip后的轴方向）。

修改后的代码：

library(ggplot2)
library(ggthemes) # 需要加载ggthemes以使用theme_tufte

ggplot(FCXM_AA_sp, aes(x = AA, y = FCXM_no, fill = FCXM)) +
  geom_bar(stat = "identity", width = .6, color = "white", size = 0.5) +
  # 关键：用position_stack(vjust=0.5)让标签在条形块内垂直居中
  geom_text(aes(label = Specificity_uniqueaa),
            position = position_stack(vjust = 0.5),
            colour = "black", size = 4) +
  coord_flip() +
  labs(title = "FCXM vs AA") +
  theme_tufte() +
  theme(plot.title = element_text(hjust = 0.5),
        axis.ticks = element_blank()) +
  scale_fill_brewer(palette = "Dark2")

关键调整说明： #

1. 调整了aes中的轴映射顺序：先写x=AA、y=FCXM_no，再通过coord_flip转成y轴为AA、x轴为FCXM_no，逻辑更清晰。
2. 移除了固定偏移的nudge_y和不合理的hjust参数，改用position_stack(vjust = 0.5)，让ggplot自动计算每个条形块的中心位置并放置标签。
3. 移除了冗余的scale_y_discrete()和scale_x_discrete()，ggplot会自动识别离散型轴并处理。

内容的提问来源于stack exchange，提问作者Pawan Raghav