Why does geom_jitter replicates black points when we use aes colour? - r

Question

I came across this issue while analyzing my data and I was able to replicate it with the example of the official ggplot reference.
This code creates black points that seem to be the original points before jitter was applied with collors:
ggplot(mpg, aes(cyl, hwy)) +
geom_point() +
geom_jitter(aes(colour = class))
However, this code works fine, it doesn't show the black points:
p <- ggplot(mpg, aes(cyl, hwy))
p + geom_point()
p + geom_jitter(aes(colour = class))
I was thinking it may be related to geom_point printing the black dots before geom_jitter, but if this is the case, why does it work fine in the second example, which follows the same order?
This is the image of the black points

Answer 1

geom_jitter is merely a convenience function, it is calling geom_point under the hood. Because of this, your use of p + geom_point() + geom_jitter(aes(color=class)) is actually the same as
ggplot(ggplot2::mpg, aes(cyl, hwy)) +
geom_point() +
geom_point(aes(color = class), position = "jitter")
which is plotting the same points twice. You can clarify this a little by changing the color of the original points:
ggplot(ggplot2::mpg, aes(cyl, hwy)) +
geom_point(color = "red") +
geom_jitter(aes(color = class))
If you want jittered points, use either geom_point(position = "jitter") or geom_jitter(), not both.