如何访问已传递给ggplot()的数据帧? [英] How do I access the data frame that has been passed to ggplot()?

问题描述

我想将字符串N=xxx设置为图形的标题，其中xxx是作为ggplot()的data参数传递的数据帧中的观察值.在我当前的代码中，我第二次显式传递该数据帧作为sprintf()的参数，该参数在labs()中使用:

I want to set the string N=xxx as the title of my figure, where xxx is the number of observations in the data frame that I pass as the data argument to ggplot(). In my current code, I explicitly pass that data frame a second time as an argument to sprintf() which I use inside of labs():

ggplot(mtcars, aes(mpg, hp)) + 
    labs(title=sprintf("N=%i", nrow(mtcars))) + 
    geom_point()

这确实会产生所需的标题，但是它不适用于更复杂的任务:我使用dplyr管道来构造要绘制的数据框，由于这是一个耗时的过程，因此我不会不想像示例中那样再次重复管道以获取行数.

This does produce the desired title, but it won't work with more complex tasks: I use a dplyr pipe to construct the data frame that is being plotted, and as this is a time-consuming process, I wouldn't want to repeat the pipe a second time to obtain the number of rows like in the example.

那么，如何从用于修改绘图的函数的参数规范中访问作为参数传递给ggplot()的数据框?

So, how do I access the data frame that has been passed as an argument to ggplot() from within the argument specifications of the functions that are used to modify the plot?

推荐答案

mtcars %>% {
  ggplot(., aes(mpg, hp)) + 
  labs(title = paste("N =", nrow(.))) + 
  geom_point()
}

请注意，在将整个ggplot调用包装在{...}大括号中时，必须将.点代词用于ggplot(., ...)中的data参数.然后，您可以在通话中的任何位置使用.代词回叫该对象.

Note that when wrapping the whole ggplot call in {...} curly braces, you must use the . dot pronoun for the data argument in ggplot(., ...). Then you can call back that object using the . pronoun anywhere in the call.

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