如何调整ggplot2底部显示的图例及图例标题

希望将fill美学对应的图例显示在图表底部，同时实现两个要求：

• a) 图例位于图表中间（x轴标签正下方）；
• b) 图例标题与图例居中对齐。

当前图表状态：图例已放置在底部，但未居中于x轴标签正下方，且图例标题与图例未对齐。

原生成代码 #

# Libraries
library(ggplot2)
library(plyr)
library(latex2exp)

# Function to calculate
calc_wet_bulb_temperature <- function(dry_temp, RH){
  wet_temp <- dry_temp*atan(0.151977 * sqrt(RH + 8.313659)) +
                  0.00391838 * sqrt(RH^3) * atan(0.023101*RH) -
                  atan(RH - 1.676331) + atan(dry_temp + RH) - 4.686035
  return(wet_temp)
}

# Space to visualise
dry_temp   <- seq(from = -10, to = 45, by = 1)
RH         <- seq(from = 0, to = 100, by = 1)
temp_space <- expand.grid(dry_temp, RH)
names(temp_space) <- c("dry_temp", "RH")

# Calculate wet bulb temperature
temp_space <- ddply(temp_space, c("dry_temp", "RH"), summarise,
                    wet_temp = calc_wet_bulb_temperature(dry_temp, RH))

# Visualise space
## Calculate value where the colour scale should be red
#diff(range(temp_space$wet_temp))
#diff(c(range(temp_space$wet_temp)[1], 30))
#42.69626/57.88312
## Actual plot
ggplot(temp_space, aes(x = RH, y = dry_temp, fill = wet_temp)) +
  geom_tile() +
  coord_equal(expand = FALSE) +
  scale_fill_viridis_c(option = "H", 
                       values = c(0.0, 0.2, 0.4, 0.6, 0.7376289, 1.0),
                       limits = range(temp_space$wet_temp)) +
 theme(legend.position = "bottom", 
       legend.title.position = "top") +
  labs(title = "Wet - bulb temperature", 
       x = "Relative humidity", 
       y = TeX(r"( Dry temperature ($C\degree$) )"),
       fill = TeX(r"( Wet temperature ($C\degree$) )"))

只需在theme()中添加三个关键参数，即可实现需求：

• legend.justification = "center"：让图例本身以中心对齐
• legend.box.just = "center"：确保图例容器在底部居中于x轴标签正下方
• legend.title.align = 0.5：让图例标题与图例中心对齐

修改后的完整代码 #

# Libraries
library(ggplot2)
library(plyr)
library(latex2exp)

# Function to calculate
calc_wet_bulb_temperature <- function(dry_temp, RH){
  wet_temp <- dry_temp*atan(0.151977 * sqrt(RH + 8.313659)) +
                  0.00391838 * sqrt(RH^3) * atan(0.023101*RH) -
                  atan(RH - 1.676331) + atan(dry_temp + RH) - 4.686035
  return(wet_temp)
}

# Space to visualise
dry_temp   <- seq(from = -10, to = 45, by = 1)
RH         <- seq(from = 0, to = 100, by = 1)
temp_space <- expand.grid(dry_temp, RH)
names(temp_space) <- c("dry_temp", "RH")

# Calculate wet bulb temperature
temp_space <- ddply(temp_space, c("dry_temp", "RH"), summarise,
                    wet_temp = calc_wet_bulb_temperature(dry_temp, RH))

# Visualise space
## Calculate value where the colour scale should be red
#diff(range(temp_space$wet_temp))
#diff(c(range(temp_space$wet_temp)[1], 30))
#42.69626/57.88312
## Actual plot
ggplot(temp_space, aes(x = RH, y = dry_temp, fill = wet_temp)) +
  geom_tile() +
  coord_equal(expand = FALSE) +
  scale_fill_viridis_c(option = "H", 
                       values = c(0.0, 0.2, 0.4, 0.6, 0.7376289, 1.0),
                       limits = range(temp_space$wet_temp)) +
  theme(
    legend.position = "bottom", 
    legend.title.position = "top",
    legend.justification = "center",  # 图例自身居中
    legend.box.just = "center",       # 图例容器在底部居中
    legend.title.align = 0.5          # 图例标题居中对齐
  ) +
  labs(title = "湿球温度", 
       x = "相对湿度", 
       y = TeX(r"( 干球温度 ($^\circ C$) )"),
       fill = TeX(r"( 湿球温度 ($^\circ C$) )"))

参数说明 #

• legend.justification：控制图例内部的对齐基准点，设为"center"让图例整体以中心为基准对齐
• legend.box.just：控制图例所在容器在图表底部的位置，确保它精准位于x轴标签正下方的中间区域
• legend.title.align：取值0（左对齐）到1（右对齐），设为0.5即可让标题与图例的中心完全对齐

内容的提问来源于stack exchange，提问作者JAQuent