r - 使用 dplyr 选择前 n 个组，然后绘制其他变量

Question

我有一个数据集，我试图通过计算一个类别来选择前 n 个，然后使用数据集中的其他变量进行绘图——基本上是前 n 个聚合的一个级别，但需要返回到在 ggplot 中绘制的完整数据。

所以在下面的问题中，我想要两个最常见的 examName，然后根据 year 的数量绘制和 facetwrap 它们。

ap <- 
      tribble(
        ~year, ~examName,
        2014, "Statistics",
        2015, "Statistics",
        2016, "Statistics",
        2016, "Statistics",
        2016, "Statistics",
        2016, "Statistics",
        2017, "Statistics",
        2017, "Statistics",
        2017, "Statistics",
        2017, "Statistics",
        2017, "Statistics",
        2013, "Macroeconomics",
        2013, "Macroeconomics",
        2014, "Macroeconomics",
        2015, "Macroeconomics",
        2016, "Macroeconomics",
        2016, "Macroeconomics",
        2016, "Macroeconomics",
        2016, "Macroeconomics",
        2016, "Macroeconomics",
        2017, "Macroeconomics",
        2017, "Macroeconomics",
        2017, "Macroeconomics",
        2017, "Macroeconomics",
        2017, "Macroeconomics",
        2017, "Macroeconomics",
        2013, "Calculus",
        2014, "Calculus",
        2015, "Calculus",
        2016, "Calculus",
        2017, "Calculus",
        2017, "Psychology",
        2017, "Psychology",
        2017, "Psychology",
        2017, "Psychology",
        2017, "Psychology",
        2018, "Psychology",
        2018, "Psychology")


ap_top <- ap %>% 
    count(examName, sort = TRUE) %>% 
    head(2) %>% 
    inner_join(ap, by = "examName") %>% 
    select(-n)

ap_top %>% 
    count(examName, year) %>% 
    ggplot(aes(x = year, y = n, group = examName)) +
    geom_line() +
    facet_wrap(~ examName)

我的想法是让我的前 n 个，然后 inner_join 回到原始数据集。然后使用它进行绘图；本质上是使用内部连接作为过滤器。

我知道有更好的方法可以做到这一点，我希望有一个更优雅的解决方案!我洗耳恭听!给出的示例数据集(抱歉太长了)。

Answer 1

您不需要 inner_join() 我只会在单独的语句中确定前两项考试，然后对其进行过滤。

top_exams <- count(ap, examName) %>% 
  top_n(2, n) %>% pull(examName)

ap %>% 
  filter(examName %in% top_exams) %>% 
  count(year, examName) %>% 
  ggplot(aes(x = year, y = n, group = examName)) +
  geom_line() +
  facet_wrap(~ examName)