数据构建来源：

https://stackoverflow.com/questions/26917689/how-to-use-facets-with-a-dual-y-axis-ggplot

library(ggplot2)


dt.diamonds <- data.table::as.data.table(diamonds)

d1 <- dt.diamonds[,list(revenue = sum(price),
                        stones = length(price)),
                  by = c("clarity","cut")]


max_stones <- max(d1$stones)
max_revenue <- max(d1$revenue)


d2 <- gather(d1, 'var', 'val', stones:revenue) %>% 
  mutate(val = if_else(var == 'revenue', as.double(val), val / (max_stones / max_revenue)))

ggplot() +
  # 柱状图数据
  geom_bar(data = filter(d2, var == 'revenue'), 
           aes(clarity, val, fill = cut),
           col = "black",
           stat = "identity") +
  # 折线图数据
  geom_line(data = filter(d2, var == 'stones'), aes(clarity, val, group = cut),
            col = "red", size = 1) +
  geom_point(data = filter(d2, var == 'stones'), aes(clarity, val, group = cut),
            col = "blue", size = 2) +
  facet_grid(cut~.) +
  # 对调 xy 轴
  coord_flip() +
  # 绘制第二个 x 轴，且原本的 x 轴放置于最上面
  scale_y_continuous(sec.axis = sec_axis(trans = ~ . * (max_stones / max_revenue),
                                         name = 'number of stones'),
                     labels = scales::dollar,
                     position = "right",
                     expand = c(0, 0)) +
  # 原本的 y 轴放置到最左边
  scale_x_discrete(position = "bottom") +
  theme_bw() +
  theme(panel.background = element_rect(fill = "#F3F2EE", colour = "#F3F2EE"),
        panel.grid.major = element_line(size = 1, linetype = 'solid',
                                        colour = "white"), 
        panel.grid.minor = element_line(size = 1, linetype = 'solid',
                                        colour = "white"),
        axis.text.x = element_text(angle = 0, hjust = 0.5),
        axis.text.y = element_text(color = "blue"),
        # axis.text.y.right = element_text(color = "red"),
        legend.position = "bottom",
        strip.text.y = element_text(angle = 0, color = "black",
                                    face = "bold", size = 12)) +
  labs(x = "")

strip.text.y 用来控制分面后标签文本的具体信息：
比如上面 strip.text.y 换成

strip.text.y = element_text(angle = -90, color = "black",
                                    face = "bold", size = 12))

如果我想将标签放在左边，且水平放置呢？

strip.text.y.left 和 strip.placement 至关重要的参数

ggplot() +
  # 柱状图数据
  geom_bar(data = filter(d2, var == 'revenue'), 
           aes(clarity, val, fill = cut),
           col = "black",
           stat = "identity") +
  # 折线图数据
  geom_line(data = filter(d2, var == 'stones'), aes(clarity, val, group = cut),
            col = "red", size = 1) +
  geom_point(data = filter(d2, var == 'stones'), aes(clarity, val, group = cut),
             col = "blue", size = 2) +
  facet_grid(cut~.,
             switch = "y") +
  # 对调 xy 轴
  coord_flip() +
  # 绘制第二个 x 轴，且原本的 x 轴放置于最上面
  scale_y_continuous(sec.axis = sec_axis(trans = ~ . * (max_stones / max_revenue),
                                         name = 'number of stones'),
                     labels = scales::dollar,
                     position = "right",
                     expand = c(0, 0)) +
  # 原本的 y 轴放置到最左边
  scale_x_discrete(position = "bottom") +
  theme_bw() +
  theme(panel.background = element_rect(fill = "#F3F2EE", colour = "#F3F2EE"),
        panel.grid.major = element_line(size = 1, linetype = 'solid',
                                        colour = "white"), 
        panel.grid.minor = element_line(size = 1, linetype = 'solid',
                                        colour = "white"),
        axis.text.x = element_text(angle = 0, hjust = 0.5),
        axis.text.y = element_text(color = "blue"),
        # axis.text.y.right = element_text(color = "red"),
        legend.position = "bottom",
        strip.background = element_blank(),
        strip.placement  = "outside",
        strip.text.y.left = element_text(angle = 0, color = "black",
                                    face = "bold", size = 12)) +
  labs(x = "")

如果标签文本过才需要换行呢？

这里给了一个答案：

How to Fit Long Text into Ggplot2 facet Titles

解决方案，就在将分面的文本预先用 str_wrap() 函数进行分行处理

library(reshape2)
library(ggplot2)

Example<-data.frame(Year,Q1,Q2, stringsAsFactors=FALSE)

ExampleM<-melt(Example,id.vars=c("Year"))

# Helper function for string wrapping. 
# Default 20 character target width.
swr = function(string, nwrap=20) {
  paste(strwrap(string, width=nwrap), collapse="\n")
}
swr = Vectorize(swr)

# Create line breaks in Year
ExampleM$Year = swr(ExampleM$Year)

ggplot(ExampleM, aes(x=variable, fill=value)) + 
  geom_bar(position="dodge") + 
  facet_grid(~Year)