如何在R的ggplot2中正确定位X轴标签并设置物种名斜体

1. 物种名斜体设置（修正拼写+两种实现方式） #

你代码里已经用了expression(italic())实现斜体，但注意物种名拼写错误：Criptosporidium应为Cryptosporidium，修正后即可正常显示斜体。

另外推荐更灵活的ggtext包方案，支持markdown语法，无需编写复杂的expression：

• 先安装并加载ggtext：

install.packages("ggtext")
library(ggtext)

• 修改scale_x_discrete的标签为markdown格式，用<br>手动换行：

scale_x_discrete(breaks = c("crypto","kvirus","TLR3"),
                 labels = c("*Cryptosporidium*<br>parvum", "CSPV1", "CSPV1 TLR3"))

• 在theme中设置X轴文本支持markdown解析：

theme(axis.text.x = element_markdown())

2. X轴标签垂直下移 #

要将物种名对应的刻度标签向下移动，只需在theme中通过margin参数调整axis.text.x的顶部间距（t值越大，标签下移幅度越大）：

theme(
  axis.text.x = element_text(margin = margin(t = 8)), # 若用ggtext则替换为element_markdown
  # 如需下移X轴标题"Exact words searched for"，添加此行：
  # axis.title.x = element_text(margin = margin(t = 15))
)

修改后的完整代码（expression版本） #

library(ggplot2)
v_pub <- 6 #CSPV1
v_gs <- 145 #CSPV1

pub <- 4858 #Cryptosporidium parvum
gs <- 69300 #Cryptosporidium parvum

vt_pub <- 0 #CSPV1 and TLR3
vt_gs <- 11 #CSPV1 and TLR3

#### GRAPH 1 ####

names <- c(rep("crypto",2), rep("kvirus",2), rep("TLR3",2))
source <- c(rep(c("pub","gs"),3))
numbers <- c(pub, gs, v_pub, v_gs, vt_pub, vt_gs)

df <- data.frame(names, source, numbers)

myplot <- ggplot(df, aes(names, numbers, fill=source)) +
  geom_bar(stat = "identity", position = 'dodge') +
  xlab("Exact words searched for") + ylab("Number of papers") +
  theme_bw() +
  scale_fill_discrete(name="Database",
                           breaks=c("gs", "pub"),
                           labels=c("Google Scholar", "PubMed")) +
  geom_text(aes(label = numbers, group=source), position=position_dodge(width=0.9), vjust=-0.2, size=4) +
  scale_x_discrete(breaks = c("crypto","kvirus","TLR3"),
                   labels = c(expression(italic("Cryptosporidium \nparvum")),"CSPV1","CSPV1 TLR3")) +
  # 调整X轴刻度标签下移幅度
  theme(axis.text.x = element_text(margin = margin(t = 8)))
myplot

修改后的完整代码（ggtext版本） #

library(ggplot2)
library(ggtext)

v_pub <- 6 #CSPV1
v_gs <- 145 #CSPV1

pub <- 4858 #Cryptosporidium parvum
gs <- 69300 #Cryptosporidium parvum

vt_pub <- 0 #CSPV1 and TLR3
vt_gs <- 11 #CSPV1 and TLR3

#### GRAPH 1 ####

names <- c(rep("crypto",2), rep("kvirus",2), rep("TLR3",2))
source <- c(rep(c("pub","gs"),3))
numbers <- c(pub, gs, v_pub, v_gs, vt_pub, vt_gs)

df <- data.frame(names, source, numbers)

myplot <- ggplot(df, aes(names, numbers, fill=source)) +
  geom_bar(stat = "identity", position = 'dodge') +
  xlab("Exact words searched for") + ylab("Number of papers") +
  theme_bw() +
  scale_fill_discrete(name="Database",
                           breaks=c("gs", "pub"),
                           labels=c("Google Scholar", "PubMed")) +
  geom_text(aes(label = numbers, group=source), position=position_dodge(width=0.9), vjust=-0.2, size=4) +
  scale_x_discrete(breaks = c("crypto","kvirus","TLR3"),
                   labels = c("*Cryptosporidium*<br>parvum", "CSPV1", "CSPV1 TLR3")) +
  theme(
    axis.text.x = element_markdown(margin = margin(t = 8))
  )
myplot

内容的提问来源于stack exchange，提问作者wingardium_leviosa98