> library(ggplot2)> library(plyr)> library(dplyr)> library(tidyr)> anscombe_tidy <- anscombe %>%+     mutate(observation = seq_len(n())) %>%+     gather(key, value, -observation) %>%+     separate(key, c("variable", "set"), 1, convert = TRUE) %>%+     mutate(set = c("I", "II", "III", "IV")[set]) %>%+     spread(variable, value)代碼中的%>%含義是什么？