冲击图(alluvial diagram)是流程图(flow diagram)的一种，最初开发用于代表网络结构的时间变化。

ggalluvial是一个基于ggplot2的扩展包，专门用于快速绘制冲击图(alluvial diagram)，有些人也叫它桑基图(Sankey diagram)，但两者略有区别。

例1：

head(as.data.frame(UCBAdmissions), n = 12)

      Admit Gender Dept Freq
1  Admitted   Male    A  512
2  Rejected   Male    A  313
3  Admitted Female    A   89
4  Rejected Female    A   19
5  Admitted   Male    B  353
6  Rejected   Male    B  207
7  Admitted Female    B   17
8  Rejected Female    B    8
9  Admitted   Male    C  120
10 Rejected   Male    C  205
11 Admitted Female    C  202
12 Rejected Female    C  391

library(ggalluvial)
is_alluvial(as.data.frame(UCBAdmissions), logical = FALSE, silent = TRUE)
### [1] "alluvia"

ggplot(as.data.frame(UCBAdmissions),
       aes(weight = Freq, axis1 = Gender, axis2 = Dept)) +
  geom_alluvium(aes(fill = Admit), width = 1/12) +
  geom_stratum(width = 1/12, fill = "black", color = "grey") +
  geom_label(stat = "stratum", label.strata = TRUE) +
  scale_x_continuous(breaks = 1:2, labels = c("Gender", "Dept")) +
  scale_fill_brewer(type = "qual", palette = "Set1") +
  ggtitle("UC Berkeley admissions and rejections, by sex and department")

例2：

titanic_wide <- data.frame(Titanic)
head(titanic_wide)

 Class    Sex   Age Survived Freq
1   1st   Male Child       No    0
2   2nd   Male Child       No    0
3   3rd   Male Child       No   35
4  Crew   Male Child       No    0
5   1st Female Child       No    0
6   2nd Female Child       No    0

# 绘制性别与舱位和年龄的关系
ggplot(data = titanic_wide, aes(axis1 = Class, axis2 = Sex, axis3 = Age, weight = Freq)) +
  scale_x_discrete(limits = c("Class", "Sex", "Age"), expand = c(.1, .05)) +
  geom_alluvium(aes(fill = Survived)) +
  geom_stratum() + geom_text(stat = "stratum", label.strata = TRUE) +
  theme_minimal() +
  ggtitle("passengers on the maiden voyage of the Titanic",
          "stratified by demographics and survival")

### data设置数据源，axis设置显示的柱，weight为数值，
### geom_alluvium为冲击图组间面积连接并按生存率比填充分组，
### geom_stratum()每种有柱状图，
### geom_text()显示柱状图中标签