何时对 R 的 ggplot2 package 中的 geom_segment() function 的 xend 参数使用 aes 映射？

Question

I'm trying to create 32 vertical line segments using the mpg column of the mtcars data frame. All is good when I run the following code:

a <- ggplot (mtcars) 
b <- geom_segment (x = 1:32, y = 0, aes (xend = 1:32, yend = mpg))
a + b

However, when I define the xend argument outside the aes() function, it looks like this.

a <- ggplot (mtcars) 
b <- geom_segment (x = 1:32, y = 0, xend = 1:32, aes (yend = mpg))
a + b

My question is, why doesn't it work? Why can I define x and y outside the aes() function but not xend ? What does the new chart mean?

Answer 1

So, to briefly recap, the purpose of the aes() function is to link up aesthetics with scales so they can be mapped. This is also why colour = "blue" will give a blue colour outside aes() but gets mapped to a discrete colourscale if you put it inside aes() .

Scales, amongst other things, measure the limits of the data range. If you define both x and xend outside the aes() and therefore these are not directly linked to a scale, the measurement that the scales are going to make won't see these values.

You can see in the example below that using that strategy gives you an x-scale that has default limits, instead of the limits as you would measure from the data.

library(ggplot2)

a <- ggplot (mtcars) 
b <- geom_segment (x = 1:32, y = 0, xend = 1:32, aes (yend = mpg))
g <- a + b

sc <- layer_scales(g)
(sc$x$get_limits())
#> [1] 0 1

^{Created on 2020-12-10 by the reprex package (v0.3.0)}

If you play around a bit more, you can see that you can define geom_segment (aes(x = 1:32, yend =mpg), y = 0, xend = 1:32) , which doesn't link the xend to a scale, but will display correctly anyway due to the scale seeing the x mapping.