如何解决ggplot2折线图中的锯齿状图形问题？

尝试制作折线图展示不同州的监狱人口增长情况，单个州分析时图表正常，但加入多个州后，图表出现上下突变的锯齿状图形。

原代码如下：

get_jail_pop_by_states <- function(states) {
  state_growth <- incarceration_trends %>% 
    select(total_jail_pop, year, state)
  select_state <- state_growth %>% 
    filter(state == states)
  return(select_state)   
}

View(get_jail_pop_by_states("MO"))

plot_jail_pop_by_states <- function(states) {
  p <- ggplot(get_jail_pop_by_states(states), aes(x=year, y=total_jail_pop, color= state))+
    stat_summary(fun.y="mean", geom= "line") +
    xlab("Year") + 
    ylab("Total jail popultion") + 
    ggtitle("Growth of Prison Population by State")

  return(p)
}
plot_jail_pop_by_states(c("CA","WA", "OR"))

问题原因 #

1. 多州筛选逻辑错误：使用state == states匹配多个州时，R的向量匹配逻辑会导致筛选结果异常，无法正确选中所有目标州的数据。
2. 冗余的均值计算：stat_summary(fun.y="mean")会对每年的所有州数据计算均值，而非按州单独绘制折线，这直接导致了锯齿状的异常图形。

修改后的代码 #

get_jail_pop_by_states <- function(states) {
  state_growth <- incarceration_trends %>% 
    select(total_jail_pop, year, state)
  # 使用%in%匹配多个州
  select_state <- state_growth %>% 
    filter(state %in% states)
  return(select_state)   
}

plot_jail_pop_by_states <- function(states) {
  p <- ggplot(get_jail_pop_by_states(states), aes(x=year, y=total_jail_pop, color= state))+
    # 直接用geom_line绘制各州的折线，无需计算均值
    geom_line() +
    xlab("Year") + 
    ylab("Total jail population") + 
    ggtitle("Growth of Prison Population by State")

  return(p)
}
plot_jail_pop_by_states(c("CA","WA", "OR"))

修改说明 #

• 把筛选条件state == states替换为state %in% states：这是R中匹配多个取值的标准语法，能正确筛选出所有目标州的记录。
• 移除stat_summary，改用geom_line()：由于每个州每年对应唯一的监狱人口总数，直接绘制折线即可保证图形的连续性，避免均值计算带来的异常。

内容的提问来源于stack exchange，提问作者xvijerhe