Why does ggplot2 allow grouping by invalid aesthetic mappings?

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I'm having trouble understanding how grouping works in ggplot. Suppose I have a dataframe with date, value, categorical, and a dichotomous variable like so: 

library(ggplot2)

set.seed(42)
data <- data.frame(date = sample(seq(as.Date('2015/01/01'), as.Date('2020/01/01'), by="day"), 20),
                   values = rnorm(20, 0, 1),
                   categories = sample(c("a", "b", "c"), 20, replace = TRUE))
data$pre_post <- ifelse(data$date <= '2018/01/01', "pre", "post")

If I group by the 'pre_post' variable (dichotomous) using aes(group =), I get:

ggplot(data, aes(x = date, y = values, color = categories, group = pre_post)) + 
  geom_line()

enter image description here

Using aes(by =), the results change to:

ggplot(data, aes(x = date, y = values, color = categories, by = pre_post)) + 
  geom_line()

enter image description here

In my current use case, 'by=' gives me the desired results but I don't know how to explain what exactly it is doing and why. Especially since, as @markus points out that 'by=' isn't even a valid aesthetic and that replacing it with 'foo=' would do the same thing:

ggplot(data, aes(x = date, y = values, color = categories, foo = pre_post)) + 
  geom_line()

enter image description here

ggplot2 r
1 Answers

If I understand the examples and explanations in Aesthetics: grouping of the ggplot2 documention correctly, the group aesthetic maps a different line for each subject.

group = pre_post seems to take precedence over the grouping by the color aesthetic enforcing to draw a single line connecting all "pre" data points and another line connecting all "post" data points.

In order to get six different lines for each combination of categories and pre_post you may follow Axeman's suggestion to use interaction():

ggplot(data, aes(x = date, y = values, color = categories, group = interaction(categories, pre_post))) + 
  geom_line()

enter image description here

or you may use a different aesthetic like linetype:

ggplot(data, aes(x = date, y = values, color = categories, linetype = pre_post)) + 
  geom_line()

enter image description here

Using an undefined aesthetic like by or foo causes the data to be grouped as well but without effect on the aesthetical appearance.

However, grouping a line chart may lead to a loss of information as can be seen here:

ggplot(data, aes(x = date, y = values, color = categories)) + 
  geom_line()

enter image description here

Without grouping by pre_post the blue line extends into 2018 which was not visible on the previous plots.

To avoid the loss of information I prefer to plot the data points as well as the lines in order to mark the end points of the line segments or to show a single data point:

ggplot(data, aes(x = date, y = values, color = categories, linetype = pre_post, shape = pre_post)) + 
  geom_line() +
  geom_point() +
  geom_label(aes(label = date), data = data[data$categories == "c" & data$pre_post == "post", ],
             hjust = -0.1, vjust = -0.1, show.legend = FALSE)

enter image description here

geom_label() is optional and only used in this post to highlight the single data point.

user_38170040
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