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I don't know about group option in ggplot function
I understand first code and graph meaning
But I don't understand second code and graph meaning
1) When second code is useful
2) When second graph is useful

ggplot documentation(https://ggplot2.tidyverse.org/reference/aes_group_order.html) explain group option is that The group aesthetic maps a different line for each subject
★3) How connect line by each subject. I Think each subject is just one point so each subject can't be line

First code and graph
ggplot(ChickWeight, aes(x=Time, y=weight, colour = Diet, group = none)) + geom_line()

enter image description here

Second code and graph
ggplot(ChickWeight, aes(x=Time, y=weight, colour = Diet, group = Chick)) + geom_point()

enter image description here

specialscene
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    Have you run these? You should have gotten an error with `group=none` because there is no column called `none` in the data. However, try running with `group="none"` (or `group="whatever"`). This results in ggplot treating all of the data as coming from a single group, so `geom_line` will draw a single line connecting all the points. In your second example, change from `geom_point` to `geom_line` and you'll see that with `group=Chick` you get a separate line for each unique value of `Chick`. – eipi10 Oct 17 '19 at 16:27
  • [This SO question](https://stackoverflow.com/questions/39878813/ggplot-geom-bar-meaning-of-aesgroup-1/39879232#39879232) might also be helpful. – eipi10 Oct 17 '19 at 16:28

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